Unsorted cleanup (#6352)

# About the pull request

Cleans up some duplicate, unused, and/or obsolete (due to native DM
implementation) code in `unsorted.dm`.

Removed (unused):
- `is_hot`
- `sign`
- `rename_self`
- `get_sorted_mobs` (`sortmobs` is the one you're thinking of)
- `getxenos`
- `getpreds`
- `gethumans`
- `getsurvivors`
- `getertmembers`
- `get_holograms`
- `sortxenos`
- `sortpreds`
- `sorthumans`
- `sortsurvivors`
- `sortertmembers`
- `blockhollow`
- `get_random_zero_sum_variances`
- `IsValidSrc`
- `convert_to_json_text`

Replaced (functionally identical):
- `InRange` to `ISINRANGE`
- `between`, `dd_range` to `clamp` (only difference was argument order)
- `check_bitflag` to `HAS_FLAG`

Replaced (special):
- `getsynths`, `sortsynths` were only used once for MOOC, which only
really needed `GLOB.human_mob_list` filtered by `issynth()`

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# Explain why it's good for the game

Less duplication leads to greater maintainability.
# Testing Photographs and Procedure
Boots without issue.


# Changelog
No player-facing changes.

code/__HELPERS/lists.dm

@@ -35,7 +35,7 @@
 /proc/listgetindex(list/list,index)
 	if(istype(list) && list.len)
 		if(isnum(index))
-			if(InRange(index,1,list.len))
+			if(ISINRANGE(index,1,list.len))
 				return list[index]
 		else if(list[index])
 			return list[index]