Fix deleting copied and modified writing systems in config

This fixes https://jira.sil.org/browse/WS-281 "[Linux] WS Config Tool - Identifier argument out of range".  Looking at the code, I can't see why this would be limited to Linux.
I struggled with this remark because StoreID appears out of the blue. At least to me it would be clearer by changing the order of words so that the "actual key" gets more emphesis (but that might be caused by my non-native English perception): The actual key for IWritingSystemRepository methods is StoreID. ...