ast: Remove ProgramIDOf(ast::Type*)

This was a stub implementation copypasta'd from the semantic types.
By removing this, ProgramIDOf(ast::Node*) will be used instead, and these checks will actually work.

Bug: tint:724
Change-Id: I72f2fc5d89d81fb485b3549cb8138517c83c9482
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49752
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/ast/type.h

@@ -134,13 +134,6 @@ class Type : public Castable<Type, Node> {
   Type(ProgramID program_id, const Source& source);
 };
 
-/// @returns the ProgramID of the given type.
-inline ProgramID ProgramIDOf(const Type*) {
-  /// TODO(crbug.com/tint/724): Actually implement this once we split the `type`
-  /// namespace into ast::Type and sem::Type.
-  return ProgramID();
-}
-
 }  // namespace ast
 }  // namespace tint