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>

The only flag is EVP_MD_CTX_FLAG_NO_INIT and no good can possibly come of
anyone outside EVP_PKEY_HMAC calling it. (And indeed no one calls it.
EVP_MD_CTX_set_flags has a caller in wpa_supplicant, but it uses
EVP_MD_CTX_FLAG_NON_FIPS_ALLOW which we don't define. The call is guarded by a
pair of ifdefs for some FIPS mode wpa_supplicant.)

Change-Id: I70ab8ffa646f3f75dfa4d37c96b9e82448ff1e40
Reviewed-on: https://boringssl-review.googlesource.com/4971
Reviewed-by: Adam Langley <agl@google.com>

Enough code fails to check their return codes anyway. We ought to make
it official.

Change-Id: Ie646360fd7073ea943036f5e21bed13df7e1b77a
Reviewed-on: https://boringssl-review.googlesource.com/4954
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)