Remove unnecessary NNAPI header files

This change removes the following header files, which were created
temporarily while NNAPI utility code was being re-organized, and are no
longer needed:
* include/AidlBufferTracker.h
* include/AidlHalInterfaces.h
* include/AidlHalUtils.h
* include/AidlValidateHal.h

Bug: 191442336
Test: mma
Test: atest neuralnetworks_utils_hal_*
Change-Id: I895d0bf6b5fd0ec4ef2977da0ecfe2a106e44083