Always enable SSL_MODE_RELEASE_BUFFERS.

There's no real need to ever disable it, so this is one fewer configuration to
test. It's still disabled for DTLS, but a follow-up will resolve that.

Change-Id: Ia95ad8c17ae8236ada516b3968a81c684bf37fd9
Reviewed-on: https://boringssl-review.googlesource.com/4683
Reviewed-by: Adam Langley <agl@google.com>