Simplify use-package setup (built-in since Emacs 29)

Remove manual installation and require of use-package since
it's built into Emacs 29+.

Author: bbatsov

core/prelude-packages.el

@@ -56,11 +56,7 @@
 (unless package--initialized
     (package-initialize))
 
-;; install & enable use-package
-(unless (package-installed-p 'use-package)
-  (package-install 'use-package))
-
-(require 'use-package)
+;; use-package is built-in since Emacs 29
 (setq use-package-verbose t)
 
 (defvar prelude-packages

modules/prelude-vertico.el

@@ -30,7 +30,6 @@
 ;; Boston, MA 02110-1301, USA.
 
 ;;; Code:
-(require 'use-package)
 
 ;; Enable vertico
 (use-package vertico