bpo-37705: Remove stray PC/errmap.mak

After pythonGH-15623 deleted `generrmap.c`, a related mak-file stopped
working. The mak contains generrmap-related rules only so it should be
removed altogether.

Further search for `errmap\.mak|generrmap` regex through content
of CPython files shows no dangling reference left.

Author: arhadthedev