Removed duplicate NSString NSURL conversion functions

Currently there's a duplication of the QString <-> NSString and
QUrl <-> NSURL functions.
This patch removes the private implementation and updates the code to
to use QtCore's functions directly.

Change-Id: I8fa613ac6753cc16fedd208f676f39ee2fec6bfa
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>