linking: remove redundant 'dont_move' parameter

The target.dont_move property is only meant to be used with target.object metadata property, not smart filters metadata properties. This was probably left accidentally unused when designing a solution for #524 (closed) involving smart filters.

If we don't want the smart filter's target to change when its metadata changes, maybe we should consider adding a new filter.smart.dont-move property... but I am not sure if this will be useful.

Fixes #558 (closed)