Let the processing thread stop the process

Fixes exception where the processing thread would try to read stdout
after the process had stopped, but before the processing thread stopped

qspectrumanalyzer/backend.py

@@ -17,7 +17,6 @@ class RtlPowerBaseThread(QtCore.QThread):
 
     def stop(self):
         """Stop rtl_power thread"""
-        self.process_stop()
         self.alive = False
         self.wait()