Remove docgen from list of default packages, don't deprecate the --de…

…fault-backend-packages option.

In https://rbcommons.com/s/twitter/r/3972/ we added this option but left it deprecated.  As per conversation on the pants-devel@ list, we don't want pants to be useless by default, so this change will allow this option to stay in place permanently, with the caveat that we are removing docgen from the list (mostly tasks only used to publish the Pants doc site.)

- Removes the docgen backend from the list of --default-backend-packages
- Removes the deprecation code around --default-backend-packages
- Updates our release scripting to add pants.backend.docgen back in for testing purposes
- Remove the `page()` target for the root README.md file to allow contrib packages to perform integration tests that don't bake in the docgen backend

Testing Done:
CI green at https://travis-ci.org/pantsbuild/pants/builds/136449213

Bugs closed: 3572

Reviewed at https://rbcommons.com/s/twitter/r/3988/

BUILD

@@ -1,4 +0,0 @@
-# Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
-# Licensed under the Apache License, Version 2.0 (see LICENSE).
-
-page(name="readme", source="README.md")

build-support/bin/publish_docs.sh

@@ -64,7 +64,7 @@ function do_open() {
 
 # generate html from markdown pages.
 ${PANTS_EXE} markdown --fragment \
-  src:: examples:: contrib:: //:readme \
+  src:: examples:: contrib::  \
   testprojects/src/java/org/pantsbuild/testproject/page:readme || \
     die "Failed to generate HTML from markdown'."