[wasm] Drop obsolete {CodeSpaceMemoryModificationScope}.

This has been functionally obsolete for a while now. From a performance
point of view it also became obsolete, because the only on-heap {Code}
object being generated within this scope is the start function wrapper.

R=clemensh@chromium.org
BUG=v8:6792

Change-Id: I978488fbd8d26b55d957d56449c5ff021b888ce1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1532320Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60374}

src/wasm/module-instantiate.cc

@@ -6,7 +6,6 @@
 
 #include "src/asmjs/asm-js.h"
 #include "src/conversions-inl.h"
-#include "src/heap/heap-inl.h"  // For CodeSpaceMemoryModificationScope.
 #include "src/property-descriptor.h"
 #include "src/utils.h"
 #include "src/wasm/js-to-wasm-wrapper-cache-inl.h"
@@ -241,8 +240,6 @@ MaybeHandle<WasmInstanceObject> InstanceBuilder::Build() {
   SanitizeImports();
   if (thrower_->error()) return {};
 
-  // TODO(6792): No longer needed once WebAssembly code is off heap.
-  CodeSpaceMemoryModificationScope modification_scope(isolate_->heap());
   // From here on, we expect the build pipeline to run without exiting to JS.
   DisallowJavascriptExecution no_js(isolate_);
   // Record build time into correct bucket, then build instance.