cutils: Remove aarch64 buffer zero checking

The revised integer version is 4 times faster than the neon version
on an AppliedMicro Mustang.  Even with hand scheduling and additional
unrolling I cannot make any neon version run as fast as the integer.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>