Remove some comments regarding caching protected buffers

We thought we'd be able to share buffers across contexts but by trial
and error (i.e. the bugs listed below) we've found that we cannot.

Bug: 242266174
Bug: 294357792
Bug: 271907068
Bug: 264475961
Bug: 267295152
Test: These are just comment changes.
Change-Id: I52e1d831f2d195cddb21c2e7a1e820059fe2a137