[cleanup] Remove forward declaration for dead ContextSlotCache

Change-Id: Iacfa08afd809f5edba8e5e4e45ebe6e9c87b9814
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1660625
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62204}

src/execution/isolate.h

@@ -70,7 +70,6 @@ class CodeTracer;
 class CompilationCache;
 class CompilationStatistics;
 class CompilerDispatcher;
-class ContextSlotCache;
 class Counters;
 class Debug;
 class DeoptimizerData;