Revert "configure: Enable GCC vectorization on ≥4.9 on x86"

This reverts commit cb8646af.

This change has brough more issues than benefits, between compilation
time failures depending on flags used and code miscompilation causing
runtime crashes.

See the "[PATCH 2/2] configure: Enable GCC vectorization on ≥4.9"
thread in the ffmpeg-devel mailing list for the relevant discussion.