perf: do not force root on Android in stop()

perf instrument was forcing killall() to run as root on Android devices.
This constraint was preventing perf from being used on unrooted devices.
However, it appears that it is possible for killall() to succeed on at
least some devices as a regular user.

This commit removes the constraint. Since killall() will default to
running as root whenever possible, the instrument will still behave
correctly on rooted Android devices where root is required.

wlauto/instrumentation/perf/__init__.py

@@ -106,8 +106,7 @@ class PerfInstrument(Instrument):
             self.device.kick_off(command)
 
     def stop(self, context):
-        as_root = self.device.platform == 'android'
-        self.device.killall('sleep', as_root=as_root)
+        self.device.killall('sleep')
 
     def update_result(self, context):
         for label in self.labels: