Remove getBoundTexture().

GetBoundTexture checks the bound and active texture,
and we should clear the id from all bound textures
(regardless of whether they're active or not).
Remove the check and getBoundTexture() method,
and just always call caches.unbindTexture(texture.id).

CRs-fixed: 681102

Change-Id: I481386394f382519bb7ea2700c4e808e00474c70