drop field for (essentially) unused tag water_volume

maybe this was intended for the tag `water_tank:volume`, but that one is also used pretty rarely on hydrants

Author: tyrasd

data/fields/water_volume.json

@@ -12,8 +12,7 @@
         "fire_hydrant/pressure",
         "fire_hydrant/position",
         "level",
-        "survey/date",
-        "water_volume"
+        "survey/date"
     ],
     "geometry": [
         "point",