Tune css-mode regexp

* lisp/textmodes/css-mode.el (css--font-lock-keywords):
Omit unnecessary \(?: \) in regexp.  Suggested by Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2019-03/msg01042.html

lisp/textmodes/css-mode.el

@@ -892,7 +892,7 @@ cannot be completed sensibly: `custom-ident',
     (,(concat "@" css-ident-re) (0 font-lock-builtin-face))
     ;; Selectors.
     ;; Allow plain ":root" as a selector.
-    ("^[ \t]*\\(:root\\)\\(?:[\n \t]*\\){" (1 'css-selector keep))
+    ("^[ \t]*\\(:root\\)[\n \t]*{" (1 'css-selector keep))
     ;; FIXME: attribute selectors don't work well because they may contain
     ;; strings which have already been highlighted as f-l-string-face and
     ;; thus prevent this highlighting from being applied (actually now that