bpo-46659: Update the test on the mbcs codec alias (GH-31168)

encodings registers the _alias_mbcs() codec search function before
the search_function() codec search function. Previously, the
_alias_mbcs() was never used.

Fix the test_codecs.test_mbcs_alias() test: use the current ANSI code
page, not a fake ANSI code page number.

Remove the test_site.test_aliasing_mbcs() test: the alias is now
implemented in the encodings module, no longer in the site module.