Remove unused method 'string_to_bool' from utils

The method 'string_to_bool' in utils is not used.
Furthermore, if we need this method in the future,
we should use 'bool_from_string' from oslo cliutils
to replace it.

partially implements blueprint common-client-library-2

Change-Id: I82af6e358d7082222fe7a09c8f27351189a82fbc