Revert "Add flush() to SkImage."

This reverts commit d0503a72ac4293562a8e88be33633ec0cf55c3d4.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Add flush() to SkImage.
> 
> This allows a client to ensure all uses of a texture-backed image have
> been flushed.
> 
> Does nothing if the image isn't texture-backed.
> 
> The implementation adds support for triggering a flush if any of a set
> of proxies are used rather than just a single proxy.
> 
> Change-Id: I358882d9737e63c6e69b924c0767f49b8f8f36ec
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212405
> Commit-Queue: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=egdaniel@google.com,bsalomon@google.com,robertphillips@google.com

Change-Id: Ie376bf4225307f45b8fb3eb4a63bf84702365797
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212884
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
25 files changed