bpo-40939: Remove the old parser (GH-20768)

This commit removes the old parser, the deprecated parser module, the old parser compatibility flags and environment variables and all associated support code and documentation.

setup.py

@@ -917,9 +917,6 @@ class PyBuildExt(build_ext):
         # select(2); not on ancient System V
         self.add(Extension('select', ['selectmodule.c']))
 
-        # Fred Drake's interface to the Python parser
-        self.add(Extension('parser', ['parsermodule.c']))
-
         # Memory-mapped files (also works on Win32).
         self.add(Extension('mmap', ['mmapmodule.c']))