config: freeze_support is not a plugin, remove from default_plugins

There is no need to inspect this module as a plugin, it only contains an
API function.

Author: bluetech

src/_pytest/config/__init__.py

@@ -269,7 +269,6 @@ def directory_arg(path: str, optname: str) -> str:
     "junitxml",
     "doctest",
     "cacheprovider",
-    "freeze_support",
     "setuponly",
     "setupplan",
     "stepwise",