Remove lines that hit ActivityMonitor on destory.

 * API document does not say ActivityMonitor is hit by onDestory.
 * We have not incremented mHits counter onDestory so far.
 * The change does not break android.app.cts.InstrumentationTest and it
   fixes the flakiness of the CTS.

BUG=29925081
TEST=android.app.cts.InstrumentationTest

Change-Id: Id0f9f5dc61e108d2c491734f089a29f0d31bde2e