Removed unused SkSLPosition

Change-Id: Icdeb0e3efce2cd9ba697edebe1c3a831144c7613
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/451256
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
7 files changed