completion: fix unused import warning

And without duplicating the conditional to import strutils.

src/completion/completion.nim

@@ -1,4 +1,4 @@
-import std/[os, strformat, strutils]
+import std/[os, strformat]
 import pkg/supersnappy
 import ../cli
 
@@ -9,7 +9,9 @@ proc readCompletions: array[Shell, string] =
     # When cross-compiling for Windows from Linux, replace the `\\` DirSep with `/`.
     var path = completionsDir / &"configlet.{shell}"
     when defined(windows) and defined(zig) and staticExec("uname") == "Linux":
-      path = path.replace('\\', '/')
+      for c in path.mitems:
+        if c == '\\':
+          c = '/'
     result[shell] = staticRead(path).compress()
 
 proc completion*(shellKind: Shell) =