Remove VMRuntime.concurrentGC

Any call would violate the precondition of the ART routine, which
has always required that it only be invoked from the GC daemon thread.

There are no known uses, and any uses would be incorrect. And its
existence interferes with the companion ART CL.

Bug: 181351667
Test: Build and boot AOSP.
Change-Id: I2551938ebba220094b18bca5590039193f60bf78
1 file changed