Removes a deprecation warning whose removal was blocked by plugin code (

pantsbuild#18722)

docs/markdown/Writing Plugins/common-plugin-tasks/plugin-upgrade-guide.md

@@ -35,6 +35,11 @@ The lazy API is useful, for example, when you only want to `Get` that output typ
 We added syntax in 2.17 to now use `Get(OutputType)`, whereas before you had to do `Get(OutputType, OutputTypeRequest)` or (as of 2.15) `Get(OutputType, {})`. So, these `OutputTypeRequest` types are now redudent and deprecated in favor of simply using `Get(OutputType)`.
 
 
+### `EnvironmentBehavior.UNMIGRATED` is no longer available
+
+Following the deprecation cycle in 2.15, all `Goal`s need to set `EnvironmentBehavior.LOCAL_ONLY` or `EnvironmentBehavior.USES_ENVIRONMENTS`.
+
+
 2.16
 ----
 

src/python/pants/engine/goal.py

@@ -9,7 +9,6 @@
 
 from typing_extensions import final
 
-from pants.base.deprecated import deprecated_conditional
 from pants.engine.unions import UnionMembership
 from pants.option.option_types import StrOption
 from pants.option.scope import ScopeInfo
@@ -83,18 +82,11 @@ class List(Goal):
     """
 
     class EnvironmentBehavior(Enum):
-        """Indicates that a goal's behavior with respect to environments has not been considered.
+        """Indicates that the goal will always operate on the local environment target.
 
-        If set, will trigger a deprecation warning. If the desired behavior is to stay pinned to
-        defaults, changing to `LOCAL_ONLY` will silence the warning for this goal.
+        This is largely the same behavior as Pants has had pre-2.15.
         """
 
-        UNMIGRATED = 1
-
-        """ Indicates that the goal will always operate on the local environment target.
-
-        This is largely the same behavior as Pants has had pre-2.15. Set to this value to silence
-        the deprecation warning that arises from using `UNMIGRATED`."""
         LOCAL_ONLY = 2
 
         f""" Indicates that the goal chooses the environments to use to execute rules within the goal.
@@ -114,17 +106,10 @@ class EnvironmentBehavior(Enum):
 
     See {doc_url('plugin-upgrade-guide')}.
     """
-    environment_behavior: ClassVar[EnvironmentBehavior] = EnvironmentBehavior.UNMIGRATED
+    environment_behavior: ClassVar[EnvironmentBehavior]
 
     @classmethod
     def _selects_environments(cls) -> bool:
-        deprecated_conditional(
-            lambda: cls.environment_behavior == Goal.EnvironmentBehavior.UNMIGRATED,
-            "2.17.0.dev4",
-            f"Setting `Goal.environment_behavior=EnvironmentBehavior.UNMIGRATED` for `Goal` "
-            f"`{cls.name}`",
-            hint=f"See {doc_url('plugin-upgrade-guide')}\n",
-        )
         return cls.environment_behavior == Goal.EnvironmentBehavior.USES_ENVIRONMENTS
 
     @final