How to hide/remove Rhythmbox Now playing pop-up/notification?

The infamous popup you are referring to is the Ubuntu notification system.

You'll see that with a variety of applications - weather apps, empathy as well as plugging in external devices.

However if you specifically dont like rhythmbox displaying a notification about which song is playing, it is simple enough to disable it.

From the rhythmbox global menu choose Edit-Plugins or Tools > Plugins, scroll down and deselect Notification

N.B. The global menu can be revealed by hovering your mouse at the top of the screen over the top of the panel which is called the menubar.

Just to add to @fossfreedomg's comment: This is how I did it in Ubuntu 14.04 (trusty)

gsettings set org.gnome.rhythmbox.plugins active-plugins "['artsearch', 'audiocd', 'audioscrobbler', 'cd-recorder', 'daap', 'dbus-media-server', 'generic-player', 'ipod', 'iradio', 'mmkeys', 'mpris', 'mtpdevice', 'power-manager']"

You can list your current settings via:

gsettings list-recursively org.gnome.rhythmbox.plugins