The asm files bn/asm/x86* weren't actually used.

(This appears to be the case with upstream too, it's not that BoringSSL
is missing optimisations from what I can see.)

Change-Id: I0e54762ef0d09e60994ec82c5cca1ff0b3b23ea4
Reviewed-on: https://boringssl-review.googlesource.com/1080
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>