bpo-37705: Remove orphaned PC/errmap.mak (GH-29724)

After GH-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.

Since generrmap is already effectively removed, this pull request contains no blurp.