Fixes the damage caused by the HE OB (#3864)

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# About the pull request

Fixes issue #3863

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# Explain why it's good for the game

This PR fixes an unintentional nerf to the HE OB.

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# Testing Photographs and Procedure
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functioning successfully, ideally including edge cases. -->


BEFORE:
Mature Queen, Praetorian, Crusher, Ravager at the edge of the screen
from a HE OB

![image](https://github.com/cmss13-devs/cmss13/assets/8739855/78e6581c-1a97-4fa5-b86a-2a2f58b6fcf3)
Prae gets obliterated, Ravager goes into crit, Crusher has 435 health,
Queen has 726 health.
AFTER:
same setup
Prae gets obliterated, Ravager gets obliterated, Crusher has 253 health,
Queen has 539 health.


# Changelog

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:cl:
fix: The HE OB now deals the correct amount of damage to xenos, before
it dealt half damage caused by xenos being forced to rest before it hit.
/:cl:

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code/modules/cm_marines/orbital_cannon.dm

@@ -430,7 +430,6 @@ var/list/ob_type_fuel_requirements
 	icon_state = "ob_warhead_1"
 	shake_frequency = 3
 	max_shake_factor = 15
-	max_knockdown_time = 6
 
 	var/clear_power = 1200
 	var/clear_falloff = 400