[DCSI] Remove SharedImageFactory from SharedImageBacking.

SharedImageFactory* in SharedImageBacking is no longer needed and hence
the corresponding methods are also not needed. It was used by dawn and
the only usage was already removed in favour of using slower
readback+upload.

This CL removes all the unused code and methods.

Bug: 448962784
Change-Id: I2643e5152138d3f0b5b40881f0f7f4889a2e6d4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7021784
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: vikas soni <vikassoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527054}