Remove docgen from list of default packages, don't deprecate the option.

Followup to  https://rbcommons.com/s/twitter/r/3988/, remove the deprecated conditional from --default-backend-packages and actually remove pangs.backend.docgen.

Testing Done:
CI running at https://travis-ci.org/pantsbuild/pants/builds/136816686

Bugs closed: 3582

Reviewed at https://rbcommons.com/s/twitter/r/3996/

build-support/bin/release.sh

@@ -78,6 +78,7 @@ function execute_packaged_pants_with_internal_backends() {
     --no-verify-config \
     --pythonpath="['pants-plugins/src/python']" \
     --backend-packages="[ \
+        'pants.backend.docgen', \
         'internal_backend.optional', \
         'internal_backend.repositories', \
         'internal_backend.sitegen', \

src/python/pants/bin/options_initializer.py

@@ -11,7 +11,6 @@
 import pkg_resources
 
 from pants.base.build_environment import pants_version
-from pants.base.deprecated import deprecated_conditional
 from pants.base.exceptions import BuildConfigurationError
 from pants.bin.extension_loader import load_backends_and_plugins
 from pants.bin.plugin_resolver import PluginResolver
@@ -150,16 +149,6 @@ def setup(self, init_logging=True):
 
     # Conditionally load backends/plugins and materialize a `BuildConfiguration` object.
     if not self._has_build_configuration():
-      deprecated_conditional(
-        lambda: not set(global_bootstrap_options.default_backend_packages).issubset(
-          global_bootstrap_options.backend_packages),
-        '1.3.0',
-        '--default-backend-packages',
-        'Add the backends you need to --backend-packages instead. To see available backends run: '
-        './pants help-advanced | grep default-backend-packages | egrep -o -m1 "default: [^)]+" . '
-        'In the future there will be no default backends, and all backends will have to be opted '
-        'into via --backend packages.'
-      )
       backends = (global_bootstrap_options.default_backend_packages +
                   global_bootstrap_options.backend_packages)
       build_configuration = self._load_plugins(self._working_set,

src/python/pants/option/global_options.py

@@ -61,23 +61,17 @@ def register_bootstrap_options(cls, register):
              help='Cache resolved plugin requirements here.')
 
     register('--backend-packages', advanced=True, type=list,
-             help='Load backends from these packages that are already on the path.')
+             help='Load backends from these packages that are already on the path. '
+                  'Add contrib and custom backends to this list.')
     register('--default-backend-packages', advanced=True, type=list,
              default=['pants.backend.graph_info',
-                      'pants.backend.docgen',
                       'pants.backend.python',
                       'pants.backend.jvm',
                       'pants.backend.codegen',
                       'pants.backend.project_info'],
-             removal_version='1.3.0',
-             removal_hint='Add the backends you need to --backend-packages instead.  To see '
-                          'available backends run: '
-                          './pants help-advanced | grep default-backend-packages | egrep -o -m1 '
-                          '"default: [^)]+" . '
-                          'In the future there will be no default backends and all backends will'
-                          'have to be opted into via --backend packages.',
-             help='Temporary helper option to ease the transition to requiring repos to '
-                  'opt-in to any needed backend.')
+             help='Load these backends by default.  These backends come distributed with Pants. '
+                  'Remove unused backends from this list to speed up execution. '
+                  'Use --backend-packages to configure additional backends with Pants.')
 
     register('--pants-bootstrapdir', advanced=True, metavar='<dir>', default=get_pants_cachedir(),
              help='Use this dir for global cache.')