[wasm] Remove dead {ThreadImpl::Block} structure.

R=clemensh@chromium.org

Change-Id: I73393686c1529a470b4c10297e88626555b9253e
Reviewed-on: https://chromium-review.googlesource.com/c/1470128Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59564}

src/wasm/wasm-interpreter.cc

@@ -1326,13 +1326,6 @@ class ThreadImpl {
     sp_t llimit() { return plimit() + code->locals.type_list.size(); }
   };
 
-  struct Block {
-    pc_t pc;
-    sp_t sp;
-    size_t fp;
-    uint32_t arity;
-  };
-
   friend class InterpretedFrameImpl;
 
   CodeMap* codemap_;