Trivial: Remove useless string_to_bool()

string_to_bool() is not used by anyone.
Furthermore, it is not well designed. It tries to convirt 't', '1'
to True, which could be confused.

So remove it. If we need something similar, let's make a better one.

Change-Id: Ic1f63480c806bf7bcc9f541fc806eed297ddf718

openstackclient/common/utils.py

@@ -258,10 +258,6 @@ def sort_items(items, sort_str):
     return items
 
 
-def string_to_bool(arg):
-    return arg.strip().lower() in ('t', 'true', 'yes', '1')
-
-
 def env(*vars, **kwargs):
     """Search for the first defined of possibly many env vars