Cleanups from the remove-native-krb5 patch

krb_srvname is actually not available anymore as a parameter server-side, since
with gssapi we accept all principals in our keytab. It's still used in libpq for
client side specification.

In passing remove declaration of krb_server_hostname, where all the functionality
was already removed.

Noted by Stephen Frost, though a different solution than his suggestion

src/backend/libpq/auth.c

@@ -129,7 +129,6 @@ static int	CheckCertAuth(Port *port);
  *----------------------------------------------------------------
  */
 char	   *pg_krb_server_keyfile;
-char	   *pg_krb_srvnam;
 bool		pg_krb_caseins_users;