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: I21978d389a8721a40e1669351575297ce61a438e
Merge "Remove unused method 'string_to_bool' from utils"