surfaceflinger: remove support for context detaching

BufferLayerConsumer::detachFromContext and
BufferLayerConsumer::attachToContext are unused.

BufferLayerConsumer::bindTextureImageLocked used to support a
special case (see libgui_test's
SurfaceTextureMultiContextGLTest.AttachAfterDisplayTerminatedSucceeds)
because of context detaching, the support can be removed as well.

Test: builds
Change-Id: I169a57ebd1b99770ce5c1cc68aa5eb097c6f1b26