Remove support for Gopher protocol (#1092)

Remove support for Gopher protocol (#1092)

Gopher code quality remains too low for production use in most
environments. The code is a persistent source of vulnerabilities and
fixing it requires significant effort. We should not be spending scarce
Project resources on improving that code, especially given the lack of
strong demand for Gopher support.

With this change, Gopher requests will be handled like any other request
with an unknown (to Squid) protocol. For example, HTTP requests with
Gopher URI scheme result in ERR_UNSUP_REQ.

Default Squid configuration still considers TCP port 70 "safe". The
corresponding Safe_ports ACL rule has not been removed for consistency
sake: We consider WAIS port safe even though Squid refuses to forward
WAIS requests:

    acl Safe_ports port 70          # gopher
    acl Safe_ports port 210         # wais