Remove `datetimepicker`

[mdziekon]

Looks like datetimepicker, a plugin for jQuery, has been unused for quite some time now, and has been replaced by native <input type="datetime-local"/> usage (see: OllisGit#205).

Since it's quite big in size, and the code still has to be maintained when refactoring stuff, it's probably best to remove it.

For future reference - it seems Firefox (on Linux) does not provide a "time selector" in the native selector popup. This is the only thing that the plugin does better - cross-compatibility between browsers. However, since it's been unused for quite some time, and users are still able to input the time (a bit more cumbersome, but still doable), so don't think reverting it to previous method is worth it.