Minor issues in MOP version of imperial fort meshes

[C3pa]

Many of them have small/invisible shapes (inherited from vanilla) that can probably be deleted for free:

• ex_imp_bridge_01⁩⁩.nif:

Also, it would probably benefit from splitting the collision shape into 3 parts:

• ex_imp_bridge_02⁩:

• ex_imp_dock_02⁩:

• ex_imp_plaza⁩:

• ex_imp_wall_tower_01⁩:

• ex_imp_keep_02⁩: