Remove oat-target* rules

This really only did anything for the 'mm' case, and was broken:

* Didn't find any apps (needed to search app/*/*.apk)
* Attempted to build framework/pm.odex instead of
  framework/oat/arm/pm.odex.
* Caused Kati to re-read all the makefiles any time the contents of
  system/app or system/framework changed.

Test: m -j installclean; m -j a (repeat until kati doesn't regenerate)
      m -j pmlib; m -j a (doesn't regenerate)
Change-Id: Icca6556f0de952b8fbc740433272766a1ce5cdc9