Remove cbk and mnemonic commands from sutil

sutil holds only the utilities wanted on every machine; it is not a
command-line front end for the packages in pkg/. cbk and bip39wordlist
remain libraries with no CLI surface. The README states the boundary.

cmd/sutil/main.go

@@ -1,5 +1,5 @@
-// Command sutil is sneak's shell utility: small tools backed by the
-// packages in sneak.berlin/go/x. Run `sutil --help` for the command list.
+// Command sutil is sneak's shell utility: a few small tools worth having on
+// every machine. Run `sutil --help` for the command list.
 package main
 
 import (