Fold away SSL_CIPHER_ALGORITHM2_VARIABLE_NONCE_INCLUDED_IN_RECORD.

It's a property of just algorithm_enc and hopefully AES-GCM will
continue to be the only true AEAD that requires this. Simpler to just
keep it in ssl_aead_ctx.c.

Change-Id: Ib7c060a3de2fa8590b2dc36c23a5d5fabff43b07
Reviewed-on: https://boringssl-review.googlesource.com/5613
Reviewed-by: Adam Langley <agl@google.com>