Skip collecting no-op DropGlue

Since 122662 this no longer gets used in vtables, so we're safe to fully
drop generating these empty functions. Those are eventually cleaned up
by LLVM, but it's wasteful to produce them in the first place.

Author: Mark-Simulacrum

compiler/rustc_monomorphize/src/collector.rs

@@ -948,10 +948,14 @@ fn visit_instance_use<'tcx>(
             bug!("{:?} being reified", instance);
         }
         ty::InstanceKind::DropGlue(_, None) => {
-            // Don't need to emit noop drop glue if we are calling directly.
-            if !is_direct_call {
-                output.push(create_fn_mono_item(tcx, instance, source));
-            }
+            // No-op drop glue never needs to be emitted.
+            // In direct calls, we skip it in codegen. In indirect calls (vtables) we place a null
+            // pointer rather than codegen'ing a pointer to the empty drop.
+            //
+            // Note that if user code casts drop_in_place to a fn(...) that's not a DropGlue, so
+            // it won't hit this branch.
+            //
+            // If we get this wrong we'll see linker errors.
         }
         ty::InstanceKind::DropGlue(_, Some(_))
         | ty::InstanceKind::FutureDropPollShim(..)