Fix deletion of cached file for policy enforcer

Policy ENFORCER's method 'clear' does not really
delete cached file as written in its docstring.

This commit adds 'delete_cached_file' to fileutils
and makes 'clear' method of ENFORCER use it.

Change-Id: I855d5f2001ac721249217d9fbd782c64af9db874
Closes-Bug: #1340843
Merge "Fix deletion of cached file for policy enforcer"