refactor(find): clean up bash-2.05 workaround for compgen -g

2>/dev/null was introduced in ad561ac. According to the commit
message, it is introduced to suppress the errors in bash
2.05. Now we only support bash >= 4.2, so we can remove the
workaround. All the other similar cases in ad561ac were already
removed.

References:
#972 (comment)
ad561ac

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

Author: akinomyoga

completions/find

@@ -36,7 +36,7 @@ _comp_cmd_find()
             return
             ;;
         -group)
-            _comp_compgen -- -g 2>/dev/null
+            _comp_compgen -- -g
             return
             ;;
         -xtype | -type)