Address TODO in testing code (i.e., cleanup create_backend_texture)

This is left over from:

https://skia-review.googlesource.com/c/skia/+/212408 (Add test for DDLs and SkSurface::flush)

Change-Id: I45797385d5ce33d4197284d53c9ec73e1a9c5e2f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212506
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed