Don't inherit from QFactoryInterface for image plugins

Don't inherit from QFactoryInterface for image plugins

QFactoryInterface is obsolete now that we have the new plugins loading
mechanism, where we can get the keys out of the plugin without
having to load it. Remove it from QImageIOHandlerFactoryInterface
as a first step of getting completely rid of it.

Change-Id: I856b149dd20131e1cdcdcb271c1a355c9e0da6ab
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>