m56d nerf - correct branch this time (#5301)

# About the pull request
Removes burst fire from the m56d
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# Explain why it's good for the game
You can no longer kill Queen in 3 seconds, now it's around 6 seconds

# Testing Photographs and Procedure

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<summary>Screenshots & Videos</summary>

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# Changelog

:cl:
balance: m56d loses burst fire
/:cl:

code/modules/cm_marines/smartgun_mount.dm

@@ -476,7 +476,6 @@
 	/// What firemodes this gun has
 	var/static/list/gun_firemodes = list(
 		GUN_FIREMODE_SEMIAUTO,
-		GUN_FIREMODE_BURSTFIRE,
 		GUN_FIREMODE_AUTOMATIC,
 	)
 	/// A multiplier for how slow this gun should fire in automatic as opposed to burst. 1 is normal, 1.2 is 20% slower, 0.8 is 20% faster, etc.