client: remove hard dependency on pam_krb5

If ipa-client-install is executed with --no-sssd, check if pam_krb5 is
available before proceeding with the install.

https://fedorahosted.org/freeipa/ticket/5557

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

freeipa.spec.in

@@ -326,7 +326,6 @@ Requires: cyrus-sasl-gssapi%{?_isa}
 Requires: ntp
 Requires: krb5-workstation
 Requires: authconfig
-Requires: pam_krb5
 Requires: curl
 # NIS domain name config: /usr/lib/systemd/system/*-domainname.service
 Requires: initscripts