Document unused function in HIDL

This costs us very little. However, there aren't many good tools to
easily detect this kind of thing. So, documenting that it can be
removed, in case it can be in the course of further cleanup.

Fixes: 132385873
Test: N/A
Change-Id: Ifc0c2a9aa88946abd74e2d1c2e7c6c2f4702df18
1 file changed