Revert "Serialize mipmaps"

This reverts commit ab5b845318a7a8fc06bd4979949149a7c482f8ec.

Reason for revert: breaks chrome -- need to use generators instead of codecs

Original change's description:
> Serialize mipmaps
> 
> We store an array of png-blobs for each level in the mip.
> 
> Change-Id: I745ee35cfec64b4b39c30650e3e5ab9b0a0ad5fd
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303023
> Commit-Queue: Mike Reed <reed@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=robertphillips@google.com,reed@google.com

Change-Id: I097cc7dfdb068fbe8043518da533195c0f115dbb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303476
Reviewed-by: Mike Reed <reed@google.com>
8 files changed