Wrong field symbol shown after a field is removed

How to reproduce:
Create a raging fire field and a blood splatter field on the same tile.
The map will show the fire symbol, not the blood symbol (correct).
Wait for some smoke to appear on that tile and disappear.
Now the blood symbol appears instead of the fire, examining the field shows that the fire is still there.

Reason is `field::removeField` (called when the smoke gets removed) sets the draw symbol to the first field type in the field-map, which (as the map is order by the field id and fd_blood comes before fd_fire) is the blood spatter.

This PR make the function search for the field with the highest priority.