Remove unused modexp512-x86_64.pl.

See upstream's c436e05bdc7f49985a750df64122c960240b3ae1.

Change-Id: I7cbe5315a769450e4630dd4e8f465cdfd45c2e08
Reviewed-on: https://boringssl-review.googlesource.com/3025
Reviewed-by: Adam Langley <agl@google.com>