[krbdev.mit.edu #5812] kill pre-beta-6 rcp support

[Ken Raeburn via RT]

Get rid of the really ancient support for encrypted user-to-user transfers in rcp.  With that, we 
also get rid of the need for krb5_set_config_files (which, while exported from libkrb5, is 
considered a private interface and therefore subject to removal) in our tree, and so can probably 
remove that function.  (But see also ticket 5784.)