[wasm] Remove unused member variable

After the v8:8689 "Split compilation in three stages" commit landed,
WasmCompilationUnit::result_ was left dangling. In builds that
noticed this (in particular certain jumbo builds), the
-Wunused-private-field warning triggered which broke the build.

Bug: v8:8689
Change-Id: Iafc56b3dc6bb53e2e8417cabce540c2fcfd3431a
Reviewed-on: https://chromium-review.googlesource.com/c/1433780
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59059}

src/wasm/function-compiler.h

@@ -100,7 +100,6 @@ class WasmCompilationUnit final {
   WasmEngine* const wasm_engine_;
   const int func_index_;
   ExecutionTier tier_;
-  WasmCode* result_ = nullptr;
 
   // LiftoffCompilationUnit, set if {tier_ == kLiftoff}.
   std::unique_ptr<LiftoffCompilationUnit> liftoff_unit_;