[cleanup] Remove unnecessary forward declaration

R=jkummerow@chromium.org

Bug: v8:9810
Change-Id: I85fa659a67b1f9f2d94cdc6107e19f9da1889fd0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1872401Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64457}

src/init/setup-isolate.h

@@ -15,10 +15,6 @@ class Code;
 class Heap;
 class Isolate;
 
-namespace interpreter {
-class Interpreter;
-}  // namespace interpreter
-
 // This class is an abstraction layer around initialization of components
 // that are either deserialized from the snapshot or generated from scratch.
 // Currently this includes builtins and interpreter bytecode handlers.