Don't remove ccache creds before storing them

Revert #6291, which attempts to suppress duplicate credentials by
calling krb5_cc_remove in krb5_cc_store_cred.  Most of our ccache
types don't implement remove_cred, and avoiding duplicate credentials
is a responsibility better suited to the ccache implementation for
atomicity reasons.

Removing this call gets rid of the misleading "Removing ... from ..."
messages in trace logs.

ticket: 7906 (new)