[wasm] Small cleanup after WasmCodeManager stitching change

The deleted lines are unnecessary, but somehow still managed
to make their way in.

Bug: 
Change-Id: If86b7e773af3149e18de59caecbf03d4558a313c
Reviewed-on: https://chromium-review.googlesource.com/804822Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49809}

src/assembler.h

@@ -54,9 +54,6 @@ namespace v8 {
 class ApiFunction;
 
 namespace internal {
-namespace wasm {
-class WasmCode;
-}
 
 // Forward declarations.
 class Isolate;
@@ -625,9 +622,6 @@ class RelocInfo {
   byte* pc_;
   Mode rmode_;
   intptr_t data_;
-  // TODO(mtrofin): try remove host_, if all we need is the constant_pool_ or
-  // other few attributes, like start address, etc. This is so that we can reuse
-  // RelocInfo for WasmCode without having a modal design.
   Code* host_;
   Address constant_pool_ = nullptr;
   friend class RelocIterator;