vtls: remove obsolete declarations of SSL backend functionality

These functions are all available via the Curl_ssl struct now, no need
to declare them separately anymore.

As the global declarations are removed, the corresponding function
definitions are marked as file-local. The only two exceptions here are
Curl_mbedtls_shutdown() and Curl_polarssl_shutdown(): only the
declarations were removed, there are no function definitions to mark
file-local.

Please note that Curl_nss_force_init() is *still* declared globally, as
the only SSL backend-specific function, because it was introduced
specifically for the use case where cURL was compiled with
`--without-ssl --with-nss`. For details, see f3b77e56

 (http_ntlm: add
support for NSS, 2010-06-27).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>