Bug 1552964 - Remove the third argument passed to run_robocop_test(). r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D31873

--HG--
extra : moz-landing-system : lando

testing/mochitest/mach_commands.py

@@ -576,7 +576,7 @@ class RobocopCommands(MachCommandBase):
             kwargs['adbPath'] = get_adb_path(self)
 
         mochitest = self._spawn(MochitestRunner)
-        return mochitest.run_robocop_test(self._mach_context, tests, 'robocop', **kwargs)
+        return mochitest.run_robocop_test(self._mach_context, tests, **kwargs)
 
 
 # NOTE python/mach/mach/commands/commandinfo.py references this function