There is a way but you need to keep Origin from re-downloading it (turn off automatic game updates) and it's not supported by EA so you'll be doing it at your own risk.
To delete the pack, go to programs(x86)\Origin_Games\thesims4 (path may vary depending on your installation) and find folders GP01 (which should be Outdoor Retreat).
Delete all instances of this folder you find within thesims4. This should delete the expansion pack from your game.
It's possible that Origin decides to re-download it on the next update but if you do the update manually, you can cancel the download right away if it does.
Again, this is at your own risk although I haven't heard of any negative effects doing this.
I have no idea why they don't offer the installation of DLC separatly. It used to be possible so I guess it has to do with Origin.
Maybe it has to do with the fact that it can lead to unexpected effects if you have a save game with certain packs present and then remove the pack.
I would post in this forum: http://answers.ea.com/t5/General-Discussion/bd-p/origin-general-discussion-en suggesting that they implement uninstalling of DLC separately.
