Remove the set_master_key and get_master_key DAL interfaces and their

Remove the set_master_key and get_master_key DAL interfaces and their
corresponding libkdb5 APIs, as they were not productively used.  In
kdb5_ldap_util, stop using the realm data's mkey field as a container
to communicate the master key to static helper functions, since the
field no longer exists.

ticket: 6749
status: open

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24162 dc483132-0cff-0310-8789-dd5450dbe970