NOTE: This extension has been archived, inactivated and is no longer supported; I don't currently use GNOME and have no way to continue development on this (besides, it never worked particularly well).
This is a simple GNOME extension to remove the world clocks section from the date menu.
Install this extension directly from https://extensions.gnome.org/extension/6973/remove-world-clocks/
To manually install this extension, copy the remove-world-clocks@codemacabre.com folder to ~/.local/share/gnome-shell/extensions.