refactor(spell): cleanup

- prefer `stdpath(data)/site/spell` instead of looking for random dirs in 'runtimepath'.
- drop unused functions `choose_directory`, `setup`, etc.

test/functional/testnvim.lua

@@ -1017,7 +1017,9 @@ end
 --- @param path string
 --- @return boolean?
 function M.mkdir_p(path)
-  return os.execute((is_os('win') and 'mkdir ' .. path or 'mkdir -p ' .. path))
+  return os.execute(
+    (is_os('win') and 'mkdir ' .. string.gsub(path, '/', '\\') or 'mkdir -p ' .. path)
+  )
 end
 
 local testid = (function()