Simplify a test in test_GetPrinterDriver().

taskmgr: Remove redundant out-of-domain checking which is already handled by PerfDataGetProcessorUsage() and PerfDataGetProcessorSystemUsage().

The implementation pointer retrieved from interface ptr wasn't being used for
anything except retrieving same interface ptr back.

It's clear that someone left it in by accident; the TABLET_FindOpenContext
is its clear successor.