Refactor machinery/anchored to default to TRUE

Deletes /obj/machinery/thruster.

Changes the density of the following abstract types:
- /obj/machinery/modular_computer
- /obj/machinery/mineral
- /obj/machinery/field
- /obj/machinery/particle_accelerator
- /obj/machinery/abductor
- /obj/machinery/telecomms

code/game/objects/structures/manned_turret.dm

@@ -6,6 +6,7 @@
 	icon = 'icons/obj/turrets.dmi'
 	icon_state = "machinegun"
 	can_buckle = TRUE
+	anchored = FALSE
 	density = TRUE
 	max_integrity = 100
 	buckle_lying = FALSE