Removes burrower from feral ERT (#5570)

# About the pull request
see title. this means that in the place of a burrower, there'll be a
hivelord or a drone instead.
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# Explain why it's good for the game
Currently, burrower doesn't really have a place in the ERT, every other
"medic" type xeno (Drone and hivelord) is able to build, emit pheros,
and weed. Meanwhile the burrower can only weed and make traps, this
isn't very useful because there's typically only 2 or 3 xenos able to
weed/emit pheros, and pheros are **Extraordinarily important** to xeno
combat, especially in XvX, while the traps take far too long to set up
for them to be very useful.
# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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# Changelog
:cl:
balance: Burrower can no longer spawn as part of the feral xeno ERT
/:cl:

code/datums/emergency_calls/feral_xenos.dm

@@ -39,7 +39,7 @@
 
 	else if(medics < max_medics)
 		medics++
-		var/picked = pick(/mob/living/carbon/xenomorph/drone, /mob/living/carbon/xenomorph/hivelord, /mob/living/carbon/xenomorph/burrower)
+		var/picked = pick(/mob/living/carbon/xenomorph/drone, /mob/living/carbon/xenomorph/hivelord)
 		new_xeno = new picked(spawn_loc)
 
 	else if(engineers < max_engineers)