Revert "Add ExternalTexture class into RenderEngine interface"

Revert submission 14086921-renderengine-external-tex

Reason for revert: Potential culprit for b/185361988
Reverted Changes:
I7796764e2:Update WaylandRenderSurface to accomodate interfac...
I13904eec4:Update Readback VTS to align with RenderEngine int...
I222c71e6e:Add ExternalTexture class into RenderEngine interf...

Change-Id: I1501890f4861a3df7ce273f1fe2ccdb275e2632c
(cherry picked from commit 617752fa)