Remove deleted pins from connection graph on symbol update

The previous attempt handling removing deleted items from connection graph is reverted. A more appropriate place is during saving the symbol back to schematics inside the SCH_SYMBOL::UpdatePins(). This function compares the edited library symbol against the schematic counterpart and already figures out pins that have to be removed. A call to remove the deleted pin from the connection graph as well was added.

Fixes #17961 (closed)