Get rid of the retain-sort-order-when-filtering logic again

For die-hard fuzzy-searching fans it's probably in the way, so taking it out
makes fuzzy filtering work better. For substring filtering it always retains the
sort order anyway.

pkg/gui/context/worktrees_context.go

@@ -19,7 +19,6 @@ func NewWorktreesContext(c *ContextCommon) *WorktreesContext {
 		func(Worktree *models.Worktree) []string {
 			return []string{Worktree.Name}
 		},
-		nil,
 	)
 
 	getDisplayStrings := func(_ int, _ int) [][]string {