Remove unused DSLExpression ctors.

We no longer have any code which constructs literals by calling
DSLExpression(value). This transitively eliminates the last bit of
code in DSLTest.

Change-Id: Iff0139f7a68b0d366711d0c241ad1ee5aa8ee086
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/697257
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
6 files changed