Remove unhelpful message from disassembly

Before:
0x352ac49c2eb4    f4  4d898df0f10600 REX.W movq [r13+0x6f1f0] (WAAT??? What are we accessing here???),r9

After:
0x352ac49c2eb4    f4  4d898df0f10600 REX.W movq [r13+0x6f1f0],r9

Change-Id: I5f9af797de3c84117fd123204d9094251d18d07b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1571618Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60939}

src/disassembler.cc

@@ -179,7 +179,7 @@ const char* V8NameConverter::RootRelativeName(int offset) const {
       SNPrintF(v8_buffer_, "external value (%s)", iter->second);
       return v8_buffer_.start();
     }
-    return "WAAT??? What are we accessing here???";
+    return nullptr;
   }
 }