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

* clean up imports and slice to remove delete

* revert buggy code

* Replace "delete" with setlen to remove depreciation warning

compiler/docgen.nim

@@ -835,15 +835,15 @@ proc docstringSummary(rstText: string): string =
   result = rstText.substr(2).strip
   var pos = result.find('\L')
   if pos > 0:
-    result.delete(pos, result.len - 1)
+    result.setLen(pos - 1)
     result.add("…")
   if pos < maxDocstringChars:
     return
   # Try to keep trimming at other natural boundaries.
   pos = result.find({'.', ',', ':'})
   let last = result.len - 1
   if pos > 0 and pos < last:
-    result.delete(pos, last)
+    result.setLen(pos - 1)
     result.add("…")
 
 proc genDeprecationMsg(d: PDoc, n: PNode): string =

compiler/jsgen.nim

@@ -29,7 +29,7 @@ implements the required case distinction.
 
 
 import
-  ast, astalgo, trees, magicsys, options,
+  ast, trees, magicsys, options,
   nversion, msgs, idents, types,
   ropes, passes, ccgutils, wordrecg, renderer,
   cgmeth, lowerings, sighashes, modulegraphs, lineinfos, rodutils,

tools/atlas/atlas.nim

@@ -9,7 +9,7 @@
 ## Simple tool to automate frequent workflows: Can "clone"
 ## a Nimble dependency and its dependencies recursively.
 
-import std/[parseopt, strutils, os, osproc, sequtils, unicode, tables, sets]
+import std/[parseopt, strutils, os, osproc, unicode, tables, sets]
 import parse_requires, osutils, packagesjson
 
 const

tools/atlas/packagesjson.nim

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