use SDFTSubRun for Slugs; remove SDFTSubRunSlug

Change-Id: Ie4b64393df378821d55f0eb96f52aff26734911c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502781
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed