Remove unused code from cc_mslsa.c

Remove unused code from cc_mslsa.c

Remove PreserveInitialTicketIdentity() and IsKerberosLogon(), as well
as the preprocessor conditionals ENABLE_PURGING and PURGE_ALL, which
have not been used in a very long time, if ever.

There was one potential callsite of IsKerberosLogon(), in
lcc_resolve(), which was disabled.  It is perfectly reasonable to want
to use the MSLSA cache on a non-domain-joined workstation, as it is
now a read-write cache type, so we need not concern ourselves whether
the logon was performed or may have been performed using kerberos.

ticket: 7989