description:
display name key/id for cube binding can not be removed once added.

build id:
2.5.2 v20100202

steps to reproduce:
1. new cube
2. insert a xtab, edit xtab binding to add display key
3. try to remove display key

actual result:
once the key is added, it can not be removed.
fixed.
verified in 2.6.0.v20100414