gcc: invoke: delete -mno-fma4 docs

gcc: invoke: delete -mno-fma4 docs

We don't document the -mno-xxx variants for other flags here, and the
paragraph here specifically says "Each has a corresponding -mno- option
to disable use of these instructions".  Drop the -mno-fma4 line.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@233144 138bc75d-0d04-0410-961f-82ee72b054a4