pcbnew: Fixup for 491098af3

Unselecting performs two functions: unselecting and unhighlighting.
Some items may be deselected but still live in the temporary highlighted
layer (e.g. selecting a footprint and exchanging it).  Since there is no
harm in unselecting an unselected item, we can allow it to proceed
without the check.

pcbnew/tools/selection_tool.cpp

@@ -1784,9 +1784,6 @@ void SELECTION_TOOL::select( BOARD_ITEM* aItem )
 
 void SELECTION_TOOL::unselect( BOARD_ITEM* aItem )
 {
-    if( !aItem->IsSelected() )
-        return;
-
     unhighlight( aItem, SELECTED, m_selection );
     view()->Update( &m_selection );