It is easy to restore that, but before you do: is your keyboard a US keyboard? There are far easier ways to type accented characters than using these popup menus.
I am Portuguese, writing in a US keyboard and one of the first thing I do is to disable the very same feature you want...
àèûçéî
where typed without my hands ever leaving the keyboard:
Option-e then letter for an acute accent - eg. Option-e followed by a for á
Option-i then letter for circumflex: âêî
Option-` then letter for grave: àè
Option-n for tilde: ãõ
Option-u for umlaut: äë
Now if you still want to reenable the click and hold method, here is what you do
Open a Terminal window, and paste the following
defaults write -g ApplePressAndHoldEnabled -bool YES
(similarly, use NO if you want to disable)
and then restart your mac.