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/utils/misc/postgresql.conf.sample

@@ -91,9 +91,8 @@
 #password_encryption = on
 #db_user_namespace = off
 
-# Kerberos and GSSAPI
+# GSSAPI using Kerberos
 #krb_server_keyfile = ''
-#krb_srvname = 'postgres'		# (Kerberos only)
 #krb_caseins_users = off
 
 # - TCP Keepalives -