libatf: remove workaround not required after atf >= 0.18 update
lib/atf/libatf-c/tests/Makefile added the -Wno-duplicate-decl-specifier
due to an issue with an old version of ATF. ATF has long since been
updated to a version with the fix so the workaround is no longer
Found during review for PR 236889.
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation