This reverts most of commit 271777f5ac. The old
ChaCha20-Poly1305, though being transitioned to the old name, should not change
in behavior. This also avoids adding a special-case to SSL_AEAD_CTX.
Also revert the name change to SSL_CIPHER_is_CHACHA20POLY1305. The one consumer
for that function doesn't need to distinguish the old and new variants, so
avoid unnecessary turbulence.
Change-Id: I5a6f97fccc5839d4d25e74e304dc002329d21b4b
Reviewed-on: https://boringssl-review.googlesource.com/6385
Reviewed-by: Adam Langley <agl@google.com>
