Remove srtp_crypto_policy_set_aes_gcm_128_8_XXXX functions from API

[pabuhler]

As noted in #440 the 8 byte tag length is explicitly not allowed according to RFC7714.

Although it is not an issues to provide these functions they are maybe misleading to users of the library that do not read RFC's first.

Would suggest marking them as deprecated and remove them from next major release.