Remove small_memcpy.

Modern clang does not appear to differentiate between memcpy
and __builtin_memcpy. The performance is identical either way.

Change-Id: I320061bf0c22184e86445ebc9ea5c23f6663db61
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/772598
Reviewed-by: Robert Phillips <robertphillips@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>