Remove a spurious TLSProxy byte in TLSv1.3

When the proxy re-encrypted a TLSv1.3 record it was adding a spurious
byte onto the end. This commit removes that.

The "extra" byte was intended to be the inner content type of the record.
However, TLSProxy was actually adding the original encrypted data into the
record (which already has the inner content type in it) and then adding
the spurious additional content type byte on the end (and adjusting the
record length accordingly).

It is interesting to look at why this didn't cause a failure:

The receiving peer first attempts to decrypt the data. Because this is
TLSProxy we always use a GCM based ciphersuite with a 16 byte tag. When
we decrypt this it actually gets diverted to the ossltest engine. All this
does is go through the motions of encrypting/decrypting but just passes
back the original data. Crucially it will never fail because of a bad tag!
The receiving party thinks the spurious additional byte is part of the
tag and the ossltest engine ignores it.
...