Remove modem log after they are added to bugreport

Delete the modem logfiles after it is added to the
bugreport zip file.

Bug: 32871032
Test: Take bugreports on the device and ensure that
      the modem log file is deleted. Also ensure that
      the generated bugreport zip file contains modem
      logs on userdebug builds

Change-Id: Ia512c11cae95e370b77024492898b3ad697fa8b0