Revert "Support GL_EXT_draw_buffers"

This reverts commit 77af4ecd3546e38e8b6ca6e0efbc6b9df65dc83a.

Reason for revert: iOS errors and bad images on Gold

Original change's description:
> Support GL_EXT_draw_buffers
> 
> Change-Id: I078aa7c42de4368602b0ef43bd7e18efbfd1e049
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212182
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

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

Change-Id: Ib8759d4a1009f9a8774365a699e211834d634cff
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212264
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
8 files changed