The old code only unbound one image unit when a texture was deleted.
The GLES specification is clear that the texture should be unbound from
all units:

  If a texture object bound to one or more image units is deleted by
  DeleteTextures, it is detached from each such image unit, as though
  BindImageTexture were called with unit identifying the image unit and
  texture set to zero.

Bug: angleproject:4310
Change-Id: I24c508660ad7eb3aa944601175803751b54ce0dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2011322Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>