Makes File::DeleteDir return true when attempting to delete a
nonexistent path.

The purpose of DeleteDir is to ensure the path doesn't exist after the
call, which is better reflected by the new return value. Additionally,
none of the current callers actually check the return value so this
won't break any existing code.

Adds a flag to File::Delete and File::DeleteDir functions to control
whether a console warning is emitted when the file or directory doesn't
exist. The flag is optional and true by default to match current behavior.