wafsamba: simplify mit kerberos detection

wafsamba: simplify mit kerberos detection

This patch removes the --with-system-mitkrb5 callback and associated
ability to store both boolean and path (string list) content.
The boolean part is self explanatory, specifying a path list
was a way to alternatively specify where krb5-config was found.
Instead now after this change to influence where krb5-config is found
the PATH variable itself should be modified before running configure.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>