[Nearby] Remove extra controller clear.

A recent CL [1] fixed the possibility of a crash in the event that
OnClose() is called multiple times. This removes logic where we clear
the controller which no longer has an effect.

1) https://chromium-review.googlesource.com/c/chromium/src/+/2548373

Bug: 1150176
Change-Id: If1f650f41c42ed64961f6fa312c0eb755def4b91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550528Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#829309}

chrome/browser/nearby_sharing/sharesheet/nearby_share_action.cc

@@ -132,5 +132,4 @@ void NearbyShareAction::OnClosing(
     nearby_ui_->RemoveObserver(this);
     nearby_ui_ = nullptr;
   }
-  controller_ = nullptr;
 }