Remove dalvik.vm.dexopt-flags (and keep precise GC)

Explanation:
* v=a (verify all classes) is default behaviour
* o=v (optimize verified classes) is default behaviour
* m=y (generate register maps) is not default, but it also enables
  precise GC, which increases .dex size.
* u=y is no longer parsed. Uni or SMP support is set during compile.

The only option that does anything (m=y) is supposed to be activated
via PRODUCT_TAGS += dalvik.gc.type-precise - use this method instead.

Change-Id: I9478fe770b015d6f3f73bb6d8b736016373a96f3