Revert most of "Refactor ChaCha20-Poly1305 AEAD nonce handling."

This reverts most of commit 271777f5ac6c7f7d1aaf95b91c5a5bef86da6363. 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>

ssl/ssl_cipher.c

diff --git a/ssl/ssl_cipher.c b/ssl/ssl_cipher.c
index 9c7f493..0ffeb5b 100644
--- a/ssl/ssl_cipher.c
+++ b/ssl/ssl_cipher.c
@@ -1388,7 +1388,7 @@
   return (cipher->algorithm_enc & (SSL_AES128GCM | SSL_AES256GCM)) != 0;
 }
 
-int SSL_CIPHER_is_CHACHA20POLY1305_OLD(const SSL_CIPHER *cipher) {
+int SSL_CIPHER_is_CHACHA20POLY1305(const SSL_CIPHER *cipher) {
   return (cipher->algorithm_enc & SSL_CHACHA20POLY1305_OLD) != 0;
 }