Cleanup operator conversion from assignment to non-assignment form.

We don't need to do string manipulation to turn `+=` into `+`. We can
make the same conversion via `op.removeAssignment()`.

Change-Id: I036ad314c161af45624efecde944bb0e596c1abd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/407816
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
1 file changed