SurfaceTexture: remove getAllocator

This change removes the SurfaceTexture::getAllocator method, as it's no
longer needed.  Proper refcounting of the Gralloc buffers is now handled
by the IGraphicBufferAlloc binder marshalling code.

Change-Id: I5cffa6ebfc1bc5828fb7ce0e0a5b2f55cd8479da