cmd: Remove comments about using the string module.

Even though we now have a `string` module, just keep the existing
IDENTCHARS definition.

- If someone doesn't already have string installed (or aren't otherwise
  importing it), this would add an extra dependency and more memory used.

- If they do, then the resulting concatenated string has to be allocated
  separately, so there's no gain from using the string.x components.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

python-stdlib/cmd/cmd.py

@@ -51,13 +51,12 @@ this means that that help by doc string feature doesn't work.
 completions have also been stripped out.
 """
 
-# import string, sys
-import sys  # MiroPython doesn't yet have a string module
+import sys
 
 __all__ = ["Cmd"]
 
 PROMPT = "(Cmd) "
-# IDENTCHARS = string.ascii_letters + string.digits + '_'
+# This is equivalent to string.ascii_letters + string.digits + '_'
 IDENTCHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_"