After the last update plasma mediacenter stop working, so I removed it. Usually I did the same thing as Virtualbox. Removed and reinstall the apps. But I can't find plasma-mediacenter from the repo. How do I reinstall plasma mediacenter?

Either you don't update, or did not use plasma-mediacenter for quite some time, it was removed from the repos early April:

To check what packages you have installed that are not in the repo:

pacman -Qm

It looks development for plasma-mediacenter plasma 5 is active again, I've been trying to build it for the kde-next repo, but it still too broken upstream to build for kf5.

ok i can't get plasma mediacenter back for now. how do i clean the recent application from menu, it's annoys me seeing the blank line on the Kicker-Recent Applications.