Fixes a bug where glide-size could become out of sync if you get unbuckled (#73150)

This fixes a bug where you could theoretically un-sync your glide-size
from your movespeed if you unbuckled. Currently, if you unbuckle, we use
a proc that's only used there which gets all of the slowdowns from all
of your movespeed modifiers. There's two problems with that:

1. We already have a variable with your movespeed (cached)
2. This proc does not check any of the conditionals that might make a
movespeed modifier not apply, such as whether or not you are flying, or
if you're immune to it for some other reason.

This means that in specific edge-cases you could mess up your
glide-size.

I have not actually seen this happen, but this seems like a better way
of doing it :)

code/modules/movespeed/_movespeed_modifier.dm

@@ -194,13 +194,6 @@ GLOBAL_LIST_EMPTY(movespeed_modification_cache)
 	for(var/id in movespeed_mod_immunities)
 		. -= id
 
-/// Calculate the total slowdown of all movespeed modifiers
-/mob/proc/total_multiplicative_slowdown()
-	. = 0
-	for(var/id in get_movespeed_modifiers())
-		var/datum/movespeed_modifier/M = movespeed_modification[id]
-		. += M.multiplicative_slowdown
-
 /// Checks if a move speed modifier is valid and not missing any data
 /proc/movespeed_data_null_check(datum/movespeed_modifier/M) //Determines if a data list is not meaningful and should be discarded.
 	. = TRUE