Remove unused code

* Remove BufferLayer::bindTextureImage
* Remove RenderEngine::bindExternalTextureBuffer. In particular, this
function was legacy back from 2012 around when SurfaceTexture was forked
into SurfaceFlinger. By now there's been some architectural changes,
including Treble enforcement of a process boundary between SF and HWC,
meaning that the "implicit synchronization" no longer really makes
sense to enforce.

Bug: 168553972
Test: builds
Test: Cuttlefish boots, can interact without visual artifacts.
Change-Id: I259160ccc6cc02fe3ac0d9b5edc4c1c72d3d582f