In an effort to bolster the iPad's multitasking abilities, iOS 11 introduced a new feature that automatically puts your most recently-used apps on the right side of the iPad dock, separated by a line. This supposedly makes it easier to switch between them, but if you're not a fan, here's how to get rid of it.
Start off by opening up the Settings app and tapping on "General".
From there, select "Multitasking & Dock".
At the bottom, tap on the toggle switch to the right of "Show Suggested and Recent Apps" to turn it off".
Boom! No more recent apps in the dock. All that will show up are only the apps that you specifically put there.
Of course, if you multitask a fair bit on your iPad, then this is probably a feature you would want. However, if you don't use your iPad for a lot of productivity stuff, something like this may not be all that useful to begin with. So it's best to disable it for a simpler user interface.