Remove unused imports, and deprecated function usage (#18663)

* clean up imports and slice to remove delete

* revert buggy code

* Replace "delete" with setlen to remove depreciation warning

tools/atlas/packagesjson.nim

@@ -1,6 +1,5 @@
 
 import std / [json, os, sets, strutils]
-import osutils
 
 type
   Package* = ref object