cmd/cue: remove obsolete TODO about empty module paths

`cue mod init` without any argument began creating a module
with path `cue.example` since https://cuelang.org/cl/1198143.
We forgot to remove this TODO as it is now resolved.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I8b2c2db13bdcb7ffda40023b3160fbcb4aa6a83d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200325
Reviewed-by: Matthew Sackman <matthew@cue.works>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/testdata/script/modinit_noargs.txtar

@@ -1,5 +1,3 @@
-# TODO we might want to deprecate or remove the ability to create
-# module file with an empty module directive.
 exec cue mod init
 env-fill want-module.cue
 cmp cue.mod/module.cue want-module.cue