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.)

mods/cnc/rules/defaults.yaml

@@ -418,7 +418,7 @@
 		TargetTypes: Ground, Water
 	BelowUnits:
 	Health:
-		HP: 500
+#		HP: 500
 	SoundOnDamageTransition:
 		DamagedSound: xplos.aud
 		DestroyedSound: xplobig4.aud