Revert "Support creation/use of mipped proxy in GrBackendTextureImageGenerator"

This reverts commit b8ad00b5a68975cafd68d6df0d66f0a01f5c07c2.

Reason for revert: Some various test failures 

Original change's description:
> Support creation/use of mipped proxy in GrBackendTextureImageGenerator
> 
> Bug: skia:
> Change-Id: I9d06780ccb2db0865100b67041c03408f2445c62
> Reviewed-on: https://skia-review.googlesource.com/61241
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Greg Daniel <egdaniel@google.com>

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

Change-Id: I28e625776352ee6f9f27e66cd5d4b149ef50a22a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/61941
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed