Remove unused grantors function in MQDescriptor am: 90d01ef298 am: b5838257e7 am: 2142e0d1da

Original change: https://android-review.googlesource.com/c/platform/system/libhidl/+/1493219

Change-Id: Ibe9b8485bb01c86337deefce007a23197c53f4f2