Remove bufferhub support from libgui.

As far as I can tell the bufferhub support in libgui was never completed,
and it is unused except for tests and the libdvr library (which is
also unused except for benchmarks and tests, which I also removed,
and its headers which I kept for now). This will allow building the
platform with newer compilers that diagnose invalid code in libpdx,
one of bufferhub's dependencies.

Bug: 246977679
Change-Id: Iff5f879c2bb5583e789a604d64638e8ce7d2b2c5