Remove bug report file on device after adb pull. (#676)

diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index 38e6eed..f549653 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -1000,6 +1000,7 @@
                 raise DeviceError(self, 'Failed to take bugreport: %s' % out)
             br_out_path = out.split(':')[1].strip()
             self.adb.pull([br_out_path, full_out_path])
+            self.adb.shell(['rm', br_out_path])
         else:
             # shell=True as this command redirects the stdout to a local file
             # using shell redirection.