[krbdev.mit.edu #8764] get_creds can add redundant cache entry for referral ticket

https://github.com/krb5/krb5/pull/912#issuecomment-494860167 
demonstrates another case where we can see a duplicate: when copying a 
ccache containing a cred with the referral realm in the service 
principal, krb5_cc_store_cred() will store a duplicate.

Since this ticket was filed, we implemented functioning remove_cred for 
all ccache types, although the FILE implementation only hides the cred.  
krb5_cc_store_cred() has a remove_cred call when storing the ticket 
server cred (mistakenly left behind by commit 
7783054742ddd807f7b2f7157d6ed81b7fb614eb) so the visible symptoms of 
this issue are no longer present.