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

(cherry picked from commit dda5396aaec315bdbcb080e42fb5cd0191f2ad72)

crypto/bn/asm/x86_64-gcc.c

@@ -64,12 +64,6 @@
  *    machine.
  */
 
-# if defined(_WIN64) || !defined(__LP64__)
-#  define BN_ULONG unsigned long long
-# else
-#  define BN_ULONG unsigned long
-# endif
-
 # undef mul
 # undef mul_add