[refactoring] remove unused imports in the compiler and in some stdlib modules

compiler/nimsets.nim

@@ -10,8 +10,7 @@
 # this unit handles Nim sets; it implements symbolic sets
 
 import
-  ast, astalgo, trees, nversion, lineinfos, platform, bitsets, types, renderer,
-  options
+  ast, astalgo, lineinfos, bitsets, types, options
 
 proc inSet*(s: PNode, elem: PNode): bool =
   assert s.kind == nkCurly