Flying mobs no longer have 3x worse damage slowdown (#91710)

## About The Pull Request

Removes a damage modifier introduced for a feature that's been long
removed.

## Why It's Good For The Game

Floating mobs are currently suffering a higher damange slowdown than
walking, this was apparently changed in this PR

https://github.com/tgstation/tgstation/pull/20860

This distinction was introduced to balance flight suits, a feature has
long been removed, therefore we no longer need this modifier.

## Changelog

🆑
del: Flying mobs no longer have 3x worse damage slowdown
/🆑

---------

Co-authored-by: Xander3359 <66163761+Xander3359@users.noreply.github.com>
Co-authored-by: MrMelbert <51863163+MrMelbert@users.noreply.github.com>

code/modules/movespeed/modifiers/mobs.dm

@@ -28,11 +28,7 @@
 	variable = TRUE
 
 /datum/movespeed_modifier/damage_slowdown
-	blacklisted_movetypes = FLOATING|FLYING
-	variable = TRUE
-
-/datum/movespeed_modifier/damage_slowdown_flying
-	movetypes = FLYING
+	blacklisted_movetypes = FLOATING
 	variable = TRUE
 
 /// Movespeed modifier applied by worn equipment.