Remove GetBlobReadOnlyPointer.

It is actually the same as GetBlobReadWritePointer. It is better not to
make the user of this function think that this function provide immutable
contents, and accidentally do something unexpected.

Tested on master branch with Marlin
and oc-dr1-daydream-dev branch with Vega
Test: buffer_hub_queue-test buffer_hub_queue_producer-test dvr_api-test
dvr_buffer_queue-test buffer_hub-test
Bug: None.
Change-Id: I1e6fd4ed1d7a1bbb611db910d35dc022bb4067a5