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.

configure

@@ -6139,11 +6139,7 @@ elif enabled ccc; then
     add_cflags -msg_disable nonstandcast
     add_cflags -msg_disable unsupieee
 elif enabled gcc; then
-    case $gcc_basever in
-        4.9*) enabled x86 || check_optflags -fno-tree-vectorize ;;
-        4.*)                 check_optflags -fno-tree-vectorize ;;
-        *)    enabled x86 || check_optflags -fno-tree-vectorize ;;
-    esac
+    check_optflags -fno-tree-vectorize
     check_cflags -Werror=format-security
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes