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
necessary.

Found during review for PR 236889.

PR: 236889
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation