making bridges indestructible for now

Making bridges indestructible.
Rationale: 
-many maps don't work when bridges are destructible (most or all of the crossings are bridges)
-bridges will be repairable in the future. At this point, an engi can come and re-open it, but in the meantime, they shouldn't be destroyed.
-if someone wants to make a map with destructible bridges, they still can, by editing the .yaml themselves. This should be the default, though.

(FWIW, bridges were not destructible in C&C 1.)