try-emacs-mua: Trim `require' advice for Emacs 25

try-emacs-mua: Trim `require' advice for Emacs 25

- Since Emacs 25 comes with `load-prefer-newer' we can remove the
  complicated variant of the advice, which implemented a poorman's
  version of that.

- Since Emacs 25 comes with the new advice mechanism, we can use
  that now for the simple variant of the advice, which just informs
  about the library that is being required.