Eliminate DSLStatement.

We no longer have any DSL remaining in SkSL. I've done a quick
sweep over the comments to remove any remaining references.

Change-Id: I2b2a7c1010c2c977aa5e3d9f4ec3a5ea853bd2d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/735519
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed