There's no good reason to do this, and it doesn't work; HMAC checks the length
of the key and runs it through the hash function if too long. The reuse occurs
after this check.

This allows us to shave 132 bytes off HMAC_CTX as this was the only reason it
ever stored the original key. It also slightly simplifies HMAC_Init_ex's
logic.

Change-Id: Ib56aabc3630b7178f1ee7c38ef6370c9638efbab
Reviewed-on: https://boringssl-review.googlesource.com/3733
Reviewed-by: Adam Langley <agl@google.com>