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.
diff --git a/configure.ac b/configure.ac
index 30856c8..3001cc8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5438,7 +5438,7 @@
 done
 
 AC_SUBST(SRCDIRS)
-SRCDIRS="Parser Parser/pegen Objects Python Modules Modules/_io Programs"
+SRCDIRS="Parser Objects Python Modules Modules/_io Programs"
 AC_MSG_CHECKING(for build directories)
 for dir in $SRCDIRS; do
     if test ! -d $dir; then