[iOS][MF] Clean unused code

Bug: 845472
Change-Id: I30ae4dd0811b60aa5509c8d3a276a62626bb454c
Reviewed-on: https://chromium-review.googlesource.com/c/1355133
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613066}

ios/chrome/browser/ui/autofill/form_input_accessory_mediator.mm

@@ -288,9 +288,9 @@
   NSString* frameID;
   if (frame) {
     frameID = base::SysUTF8ToNSString(frame->GetFrameId());
-  } else {
-    frameID = base::SysUTF8ToNSString(params.frame_id);
   }
+  DCHECK(frameID.length ||
+         !autofill::switches::IsAutofillIFrameMessagingEnabled());
 
   [self.formInputAccessoryHandler setLastFocusFormActivityWebFrameID:frameID];
   [self synchronizeNavigationControls];