crypto/bn/asm/x86_64-gcc.c: remove unnecessary redefinition of BN_ULONG

This module includes bn.h via other headers, so it picks up the
definition from there and doesn't need to define them locally (any
more?).  Worst case scenario, the redefinition may be different and
cause all sorts of compile errors.

Fixes #7227

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7287)