Check for dup layer names in the GUI, not in the internals.

Doing so in the internals keeps users from being able to swap layer
names as the dialog does each layer name change individually.

Fixes: lp:1842665
* https://bugs.launchpad.net/kicad/+bug/1842665

pcbnew/class_board.cpp

@@ -316,26 +316,7 @@ bool BOARD::SetLayerName( PCB_LAYER_ID aLayer, const wxString& aLayerName )
 
     if( IsLayerEnabled( aLayer ) )
     {
-#if 0
-        for( LAYER_NUM i = FIRST_COPPER_LAYER; i < NB_COPPER_LAYERS; ++i )
-        {
-            if( i != aLayer && IsLayerEnabled( i ) && nameTemp == m_Layer[i].m_Name )
-                return false;
-        }
-#else
-        for( LSEQ cu = GetEnabledLayers().CuStack();  cu;  ++cu )
-        {
-            PCB_LAYER_ID id = *cu;
-
-            // veto changing the name if it exists elsewhere.
-            if( id != aLayer && nameTemp == m_Layer[id].m_name )
-//            if( id != aLayer && nameTemp == wxString( m_Layer[id].m_name ) )
-                return false;
-        }
-#endif
-
         m_Layer[aLayer].m_name = nameTemp;
-
         return true;
     }