Merge IMPLEMENT_tls_meth_func and IMPLEMENT_ssl3_meth_func.

The TLS-specific hooks have been removed. We aim to no longer perform version
negotiation as a pre-processing step, so ensure the only differences to worry
about are the version, get_method hook, and the enc_data.

BUG=chromium:403378

Change-Id: I628ec6f4c50ceed01d7af8f4110b6dc95cfbe023
Reviewed-on: https://boringssl-review.googlesource.com/1841
Reviewed-by: Adam Langley <agl@google.com>