Disable multithreaded dexopt by default

* There is still a race condition here.
* Power users can re-enable this by setting
  persist.sys.dalvik.multithread to true

Change-Id: I03686bafbac1859695466fcb522bc5436ec82e0d
Remove unnecessary duplicate

* We disable multithreaded dexopt for all builds by default now.

Change-Id: I05c01c3b7911fa8e6d3e9410f5d1cc800292ebda