Mojave can’t be installed but still boots to Installer partition

I downloaded macOS Mojave from the App Store and, upon running it, it told me that my MBP (15-inch, Mid 2010, currently High Sierra) can’t be upgraded to Mojave.

Then it restarted my Mac… into the same installer again. This time, the installer complained of some missing files.

I booted back into my Mac and deleted the .app file, but the boot menu (by pressing Option during boot) are still showing 3 partitions: mac OS Installer, OSX, and Recovery HD—in that order.

The weird thing is, both mac OS Installer and Recovery HD seem to open the same thing—a menu(?) where you can reinstall Lion(!!!).

My question: How do I remove mac OS Installer from the boot menu? Or, at least, reorder the menu so that OSX comes first.

I read on the internet to rerun the installer and cancel it there—well, I’ve deleted the file so I have to download it again, but now App Store prevents me from downloading it (“This version of macOS 10.14.5 cannot be installed on this computer.”—I know! Why didn’t you say so last time!)