Revert "Spruce up SkSL slide with a much nicer editor"

This reverts commit 88c5af7ecd72090b2d2eff2aec3a28c30c72a7fb.

Reason for revert: Various input bugs in the new editor make it somewhat unusable.

Original change's description:
> Spruce up SkSL slide with a much nicer editor
>
> Pulls in a new GitHub repo that implements a syntax-highlighting code
> editor ImGui widget.
>
> Change-Id: I968e5eb827c226259eaaad2996eeaad9de592e37
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/491444
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

Change-Id: Ifa5bc87327785d107956201c4d866c7259006ec2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/492359
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed