gssproxy: Don't refresh expired delegated credentials

`mod_auth_gssapi` exports delegated credentials into `/run/ipa/ccaches`
and pass down that path as `KRB5CCNAME` env variable to WSGI worker.

GSSProxy in turn, protects these credentials from direct usage of
`ipa-api`. But the configuration of `service/ipa-api` (in particular,
'cred_store = client_keytab:/var/lib/ipa/gssproxy/http.keytab') and
default GSS name ('=None') dictates to refresh expired credentials
with the client's keytab overwriting the origin credentials with
initial credentials of keytab's principal.

Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

install/share/gssproxy.conf.template

@@ -11,7 +11,6 @@
 [service/ipa-api]
   mechs = krb5
   cred_store = keytab:$HTTP_KEYTAB
-  cred_store = client_keytab:$HTTP_KEYTAB
   allow_constrained_delegation = true
   allow_client_ccache_sync = true
   cred_usage = initiate