Remove altivec specific memory clearing

Remove altivec specific memory clearing

I've no idea how common powerpcs with 32byte cache lines are nowadays
(All powerpc's i've ever had access to had 32 byte wide cache lines
for 32bit and 128 for 64. Given that 32bit PPCs are, for all intents
and purposes, dead and the fact that altivec optimized memclear code
will fall back to regular clearing on non 32 byte cache-lined
machines, the code serves no useful purpose, so - remove it.