[diagnostics] Remove redundant DisallowGarbageCollection

And fixes a -Wshadow warning too.

Bug: v8:12244,v8:12245
Change-Id: I940600c21c81fd757794dc934eaf6e918d058bc8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3283621Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77927}

src/diagnostics/perf-jit.cc

@@ -393,8 +393,6 @@ void PerfJitLogger::LogWriteDebugInfo(Handle<Code> code,
     entry.line_number_ = info.line + 1;
     entry.column_ = info.column + 1;
     LogWriteBytes(reinterpret_cast<const char*>(&entry), sizeof(entry));
-    // The extracted name may point into heap-objects, thus disallow GC.
-    DisallowGarbageCollection no_gc;
     std::unique_ptr<char[]> name_storage;
     base::Vector<const char> name_string =
         GetScriptName(info, &name_storage, no_gc);