Self published - can you still seek lit agents?

It depends on how it sold and whether you have another book.

If a book didn't dazzle anyone when self-published, that is hardly a selling point to an agent. Only a mediocre agent is looking for mediocre books. Most published books fail, so why would someone buy a failed book?

However, modest sales in your last book might sell your next book.

My attitude is that any agent or publisher who would refuse to consider a book simply because it had already been published doesn't deserve to be allowed to consider it, because they're not living in the 21st century.

With that out of the way, yes, you can submit a previously self-published book to any agent or publisher (unless their submission guidelines specifically say they don't want such books), and no, you don't need to take it down from wherever it's on sale. You'd be foolish to take it down, because there's about a 99% chance that they'll reject it, so you'd just be killing any sales momentum it had for no good reason.

Bear in mind, though, that agents and publishers aren't likely to be interested in a self-published book unless it's already sold very well. How well is "very well"? It depends who's deciding, but you're probably looking at tens of thousands of copies. They seem to think that if you self-published a book and it didn't sell well, that means there's no audience for it (or not a big enough audience for them to bother getting involved, anyway).