[vm,dart2wasm] Remove old TODO in convert_patch

The block right above the TODO comment does exactly what the comment
describes. Remove the comment.

Tested: comment-only change, tested by existing tests.
Change-Id: Ic33bca5873a0e3deb7b5bf817ebde93dcda760be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392360
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Lasse Nielsen <[email protected]>

Author: osa1

sdk/lib/_internal/vm/lib/convert_patch.dart

@@ -44,7 +44,6 @@ class Utf8Decoder {
               (next as JsonDecoder)._reviver, this._allowMalformed)
           as dynamic/*=Converter<List<int>, T>*/;
     }
-    // TODO(lrn): Recognize a fused decoder where the next step is JsonDecoder.
     return super.fuse<T>(next);
   }
 }

sdk/lib/_internal/wasm/lib/convert_patch.dart

@@ -40,7 +40,6 @@ class Utf8Decoder {
               (next as JsonDecoder)._reviver, this._allowMalformed)
           as dynamic/*=Converter<List<int>, T>*/;
     }
-    // TODO(lrn): Recognize a fused decoder where the next step is JsonDecoder.
     return super.fuse<T>(next);
   }
 }