maint.mk: avoid redundant file name in message

maint.mk: avoid redundant file name in message

$(_sc_search_regexp) already prefixes its errors with $(ME), so
failure on one of these rules looked like:

maint.mk: maint.mk: replace strcmp calls above with STREQ/STRNEQ

* top/maint.mk (sc_prohibit_strcmp, sc_unmarked_diagnostics)
(sc_prohibit_defined_have_decl_tests, sc_const_long_option)
(sc_makefile_path_separator_check): Remove bogus $(ME).

Signed-off-by: Eric Blake <eblake@redhat.com>