Fixes shotgun calibers (#3573)

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## About The Pull Request

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Fixes the caliber strings on the beanbag, rubber, improvised and
incendiary shotgun rounds and shotguns being unable to load them as a
consequence.

Magazines check for an exact caliber match, so having unique strings
like "12ga beanbag" means the round gets rejected.

## Why It's Good For The Game

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Fixes are good
## Changelog

:cl:
fix: Shotgun calibers and shotguns being unable to load certain rounds
/:cl:

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code/modules/projectiles/ammunition/ballistic/shotgun.dm

@@ -27,15 +27,13 @@
 	name = "beanbag slug"
 	desc = "A weak beanbag slug for riot control."
 	icon_state = "beanbag"
-	caliber = "12ga beanbag"
 	custom_materials = list(/datum/material/iron=250)
 	projectile_type = /obj/projectile/bullet/slug/beanbag
 
 /obj/item/ammo_casing/shotgun/rubbershot
 	name = "rubber shot"
 	desc = "A shotgun casing filled with densely-packed rubber balls, used to incapacitate crowds from a distance."
 	icon_state = "rubber"
-	caliber = "12ga rubber"
 	projectile_type = /obj/projectile/bullet/pellet/rubbershot
 	pellets = 8
 	variance = 25
@@ -45,7 +43,6 @@
 	name = "incendiary slug"
 	desc = "An incendiary-coated shotgun slug."
 	icon_state = "incendiary"
-	caliber = "12ga incendiary"
 	projectile_type = /obj/projectile/bullet/incendiary/shotgun
 
 /obj/item/ammo_casing/shotgun/blank
@@ -59,7 +56,6 @@
 	name = "improvised shell"
 	desc = "An extremely weak shotgun shell with multiple small pellets made out of metal shards."
 	icon_state = "improvised"
-	caliber = "improvised 12ga"
 	projectile_type = /obj/projectile/bullet/pellet/improvised
 	custom_materials = list(/datum/material/iron=250)
 	pellets = 10