The memtest entries in GRUB are just annoying and useless IMO (I think they shouldn't even come preinstalled, they only confuse newbies and if you really need them you can install them later...). Is it a bad idea to remove them?
No. Memtest can always be run from optical or USB media instead.
There is no harm in removing them. I would suggesting leaving them there. Memtest is for checking your RAM. It is very easy to use. You just set it running and watch the results. Basically, a healthy RAM should show no errors.
To remove, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo chmod -x /etc/grub.d/20_memtest86+ sudo update-grub
Or just use Grub Customizer.
You can always run memtest by downloading the boot CD from memtest.