[compiler] Fix the gcc build

.. by removing the UNREACHABLE call.

Bug: v8:7790
Change-Id: Ib2f299df78ace89b17a6956da6018d108ff1a0ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3074469
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76157}

src/compiler/heap-refs.cc

@@ -1817,7 +1817,6 @@ constexpr ObjectDataKind ObjectDataKindFor(RefSerializationKind kind) {
     case RefSerializationKind::kNeverSerialized:
       return kNeverSerializedHeapObject;
   }
-  UNREACHABLE();
 }
 
 }  // namespace