This removes EVP_PKEY_HMAC and all the support code around it. EVP_MD requires
a lot of extra glue to support HMAC. This lets us prune it all away.

As a bonus, it removes a (minor) dependency from EVP to the legacy ASN.1 stack.

Change-Id: I5a9e3e39f518429828dbf13d14647fb37d9dc35a
Reviewed-on: https://boringssl-review.googlesource.com/5120
Reviewed-by: Adam Langley <agl@google.com>