Make krb_realm and krb_server_hostname be pg_hba options only, and remove

their GUCs.

In passing, noted that the pg_hba options for krb5 authentication weren't
listed at all - so add this.

src/backend/utils/misc/postgresql.conf.sample

@@ -81,10 +81,7 @@
 # Kerberos and GSSAPI
 #krb_server_keyfile = ''
 #krb_srvname = 'postgres'		# (Kerberos only)
-#krb_server_hostname = ''		# empty string matches any keytab entry
-					# (Kerberos only)
 #krb_caseins_users = off
-#krb_realm = ''
 
 # - TCP Keepalives -
 # see "man 7 tcp" for details