Prune away (almost) all of asn1_mac.h

Amazingly, asn1_GetSequence isn't completely unused? Keep that around for now
and ditch everything else. This lets us enable C4311 in MSVC which is actually
a pretty reasonable warning.

Change-Id: I43bb9206b1745e8a68224f3a435713d2a74e04ea
Reviewed-on: https://boringssl-review.googlesource.com/4256
Reviewed-by: Adam Langley <agl@google.com>