Revert "Add DSL PossibleExpression & PossibleStatement"

This reverts commit d42932a1b5b19098961b4837d54f75a52f278817.

Reason for revert: Breaking CMake and SKQP builds.

Original change's description:
> Add DSL PossibleExpression & PossibleStatement
>
> These are currently unused, but in future CLs they will be used to
> capture line number information in DSL error handling.
>
> Change-Id: Ieee730e0ad8323043437972fedb5bec471c367e4
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375069
> Reviewed-by: John Stiles <johnstiles@google.com>

TBR=brianosman@google.com,ethannicholas@google.com,johnstiles@google.com

Change-Id: I2062c9964569c9129a2145f9e45f849310129687
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375658
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
diff --git a/tests/SkSLDSLTest.cpp b/tests/SkSLDSLTest.cpp
index 0055f78..0fde847 100644
--- a/tests/SkSLDSLTest.cpp
+++ b/tests/SkSLDSLTest.cpp
@@ -44,7 +44,7 @@
         SetErrorHandler(nullptr);
     }
 
-    void handleError(const char* msg, PositionInfo* pos) override {
+    void handleError(const char* msg) override {
         REPORTER_ASSERT(fReporter, !strcmp(msg, fMsg),
                         "Error mismatch: expected:\n%sbut received:\n%s", fMsg, msg);
         fMsg = nullptr;