The BindingPointer pattern isn't necessary. Every time we delete a
Texture we call ReleaseTexImage internally. There shouldn't be any time
that we keep an orphaned Texture is Surface.

This cleans up one place where we were using the ProxyContext.

Bug: angleproject:2714
Change-Id: I3b0fd2125d02ea7545922ec6da7f487451bed871
Reviewed-on: https://chromium-review.googlesource.com/1128925
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>