Remove -ftree-vectorize from CFLAGS_GCC_OPTI

[Artoria2e5]

Both the loop and SLP vectorizers are now enabled by -O2 in gcc 12 -- that's the same as enabling -ftree-vectorize. As a result, there is probably no effect in specifying it explicitly. https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ftree-loop-vectorize

This might also mean that we are starting to accidentally vectorize the retro arches, since instead of asking for -fno-tree-XXX-vectorize, we only took -ftree-vectorize out.

P.S. GCC 12 also introduced a "very-cheap" cost model so it does not over-vectorize at -O2. https://www.phoronix.com/news/GCC-12-Auto-Vec-O2