pgcrypto: remove useless px_memset() and BF_ASM

Hi Hackers,

In the thread about centralised architecture detection, I noticed that
the BF_ASM macro in crypt-blowfish.c has never been defined to anything
but 0, and the _BF_body_r() function it would call has never existed, so
that can be got rid of.

While investigating at that, I also noticed that px_memset(), which has
the comment /* memset that must not be optimized away */, is only ever
called with zero for the value, which could be better written with
explicit_bzero() now that we have that.

Attached are patches for both.

- ilmari