This removes support code for a "stream_mac" mode only used by GOST. Also get
rid of this

   /* I should fix this up TLS TLS TLS TLS TLS XXXXXXXX */

comment next to it. It's not actually related to GOST (dates to OpenSSL initial
commit), but isn't especially helpful at this point.

Change-Id: Ib13c6e27e16e0d1fb59ed0142ddf913b9abc20b7
Reviewed-on: https://boringssl-review.googlesource.com/1281
Reviewed-by: Adam Langley <agl@google.com>