Prevent growing fwlayout files

This fixes FWNX-1057 "layout configuration files can grow with duplicate
entries".  I suppose you could argue that this reflects a Mono library
bug, in that removing an child element from an XmlElement invalidates
(and thus terminates) an iterator for an XmlNodeList involving child
nodes of that element.  But it's an easy fix in FieldWorks code.
(cherry picked from commit eb74df3b220f41838199a2b566c3a80d41b7cd7c on
develop)

Change-Id: I83a09f7c8ec7c6cbd6b39cf51017090ece9bd9c7