Removed unused Utility function in_array_insensitive

in_array_insensitive is not referenced anywhere in the code, and
causes PHPCS to fail on the Utility class as it's not in camelcase.

There does not appear to be any reason to keep this legacy function.
Merge pull request #822 from driusan/PHPCS-Utility_inarrayinsensitive

Removed unused Utility function in_array_insensitive