Remove stale TODOs about audio HALs

Both 6.0 and 7.0 can be used for launching new
devices on S due to large amount of refactorings
in the interfaces. Vendors who don't need
the new features added in V7 are free to continue
using V6.

Bug: 142480271
Test: N/A
Change-Id: I133348b9a0a1a200ba28b2d67cac92f1962a3e09