Eleven: gradle: remove obsolete dexOptions

  WARNING:DSL element 'dexOptions' is obsolete and should be removed.
  It will be removed in version 8.0 of the Android Gradle plugin.
  Using it has no effect, and the AndroidGradle plugin optimizes
  dexing automatically.

Change-Id: I24645b311fe52a10c8b0b3a3232541b3332ad4dc
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>