Do not retain paper view inside legacy interop after it has been recycled

Summary:
Changelog: [Internal]

Paper views are not designed to be recycled, that's why a new view is created each time legacy interop layer is used.
However paper view was not deallocated immediately after it was used, it was still being strongly referenced by `self.contentView`.
This doesn't cause an immediate issue because eventually when legacy interop layer does get reused, it will create a new paper view and replace the old one inside `self.contentView`, triggering deallocation of the old one. But we were retaining the paper view beyond what was necessary.

Reviewed By: mdvacca

Differential Revision: D22066022

fbshipit-source-id: 17c3b81468f50ebcc05f1a7cdf4d4b9b00965fc3

React/Fabric/Mounting/ComponentViews/LegacyViewManagerInterop/RCTLegacyViewManagerInteropComponentView.mm

@@ -87,6 +87,7 @@ static NSString *const kRCTLegacyInteropChildIndexKey = @"index";
   [_viewsToBeMounted removeAllObjects];
   [_viewsToBeUnmounted removeAllObjects];
   _state.reset();
+  self.contentView = nil;
   [super prepareForRecycle];
 }