emacs: drop obsolete emacs property

New emacs warns on every open about the old attribute. Let's assume that
everybody who uses emacs uses the latest version, so we can drop the compat.

.dir-locals.el

@@ -20,8 +20,7 @@
  (nxml-mode . ((nxml-child-indent . 2)
                (fill-column . 109)))
  (meson-mode . ((meson-indent-basic . 8)))
- (sh-mode . ((sh-basic-offset . 4)
-             (sh-indentation . 4)))
+ (sh-mode . ((sh-indentation . 4)))
  (awk-mode . ((c-basic-offset . 8)))
  (nil . ((indent-tabs-mode . nil)
          (tab-width . 8)