[NFC] Remove unused useNewWATParser bool (#7170)

We always use the new WAT parser now.

Author: tlively

src/wasm-io.h

@@ -28,10 +28,6 @@
 
 namespace wasm {
 
-// TODO: Remove this after switching to the new WAT parser by default and
-// removing the old one.
-extern bool useNewWATParser;
-
 class ModuleIOBase {
 protected:
   bool debugInfo;