libpq: Remove code for SCM credential authentication

libpq: Remove code for SCM credential authentication

Support for SCM credential authentication has been removed in the
backend in 9.1, and libpq has kept some code to handle it for
compatibility.

Commit be4585b, that did the cleanup of the backend code, has done
so because the code was not really portable originally.  And, as there
are likely little chances that this is used these days, this removes the
remaining code from libpq.  An error will now be raised by libpq if
attempting to connect to a server that returns AUTH_REQ_SCM_CREDS,
instead.

References to SCM credential authentication are removed from the
protocol documentation.  This removes some meson and configure checks.

Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/ZBLH8a4otfqgd6Kn@paquier.xyz