System.ArgumentOutOfRangeException is thrown when deleting several elements from MvxObservableCollection in a loop
## 🔙 Regression

<!--- Summary description of the regression --->
When trying to remove several elemnts form MvxObservableCollection I have System.ArgumentOutOfRangeException "Index was out of range. Must be non-negative and less than the size of the collection." thrown.

I took this lines from the logger
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at System.Collections.Generic.List\`1[T].get_Item (System.Int32 index) <0x101492340 + 0x00063> in <0c3d4da7367647c9a650750b1a4d0c53#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at System.Collections.ObjectModel.Collection`1[T].System.Collections.IList.get_Item (System.Int32 index) <0x101486ac0 + 0x0004b> in <0c3d4da7367647c9a650750b1a4d0c53#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at MvvmCross.Binding.Extensions.MvxEnumerableExtensions.ElementAt (System.Collections.IEnumerable items, System.Int32 position) <0x102afcb00 + 0x000b3> in <03f32c17c8e741219cee9c3d186bfa64#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at MvvmCross.Platforms.Ios.Binding.Views.MvxTableViewSource.GetItemAt (Foundation.NSIndexPath indexPath) <0x102ab5fc0 + 0x00053> in <03f32c17c8e741219cee9c3d186bfa64#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at MvvmCross.Platforms.Ios.Binding.Views.MvxBaseTableViewSource.GetCell (UIKit.UITableView tableView, Foundation.NSIndexPath indexPath) <0x102ab29b0 + 0x00033> in <03f32c17c8e741219cee9c3d186bfa64#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) <0x101f9da40 + 0x0002f> in <ad1be307855442e4b2cb88e33f94f908#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) <0x101f9d9b0 + 0x00043> in <ad1be307855442e4b2cb88e33f94f908#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
  at Formpipe.MeetingsPlus.MobileApplication.Ios.Presentation.Application.Main (System.String[] args) <0x101134f80 + 0x000bc> in <da3df9e23dc7422b9cbc787f70ad899e#c2280c4f5e279e441ebcc79f4b4f9d7e>:0 
### Old (and correct) behavior
Collection items are deleted without exceptions.
### Current behavior
Collection throws System.ArgumentOutOfRangeException when trying to remove 5-6th element.
### Reproduction steps
- Create MvxObservableCollection and bind it to some view
- Add several items to the collection (10-15)
- Try to remove all elements in a loop (not via RemoveRange or Clear)

**Version:** 7.1

**Platform:** :iphone: iOS

### P.S.
I have also written about System.InvalidOperationException when modifying MvxObservableCollection in a loop.
https://github.com/MvvmCross/MvvmCross/issues/4139
However, I've copied the collection calss and made OnCollectionChanged synchronous, so there is a problem somwhere else
