Remove DTLS special-cases in buffer releasing.

They date to https://rt.openssl.org/Ticket/Display.html?id=2533, but no
particularly good justification was given for them. It seems it was just a
bandaid because d1_pkt.c forgot to initialize the buffer. I went through
codesearch for all accesses to SSL3_BUFFER::buf and SSL::packet. They seem
appropriately guarded but for this one.

Change-Id: Ife4e7afdb7a7c137d6be4791542eb5de6dd5b1b6
Reviewed-on: https://boringssl-review.googlesource.com/4685
Reviewed-by: Adam Langley <agl@google.com>