Remove plugin directory per CPU capability hack

On x86, describing and probing optimized plugins is safe (thanks to the
removal of the -mmmx and -msse2 compiler flags).

On PowerPC, libvlccore is built with -maltivec. This is a bug (no
difference before or after the plugin directory hack).
Describing and probing Altivec plugins is no worse.

On ARM, NEON support is detected only at compilation time currently
for lack of a better alternative. So this is a non-issue.