How to remove MacOS and install Ubuntu
The Grub boot files installed by Ubuntu are compatible with the Mac Startup Manager. While the Mac should automatically boot Ubuntu, you can also select to always boot from Ubuntu by holding down the option key at startup, then selecting the icon label EFI Boot. After doing so, the Mac should boot Ubuntu by default.
I assume here your MacBook Pro is a newer model that would capable of 64 bit EFI booting Ubuntu and have at least 4 GB of memory. If not, then you should update your question.
I also assume here your Mac does not have the T2 chip. If your Mac has the T2 chip, then you have to remove Secure Boot and allow External Boot. Some releases of Ubuntu can not be installed on the internal drive of newer Intel Macs. If this is the case with your Mac, then you will need to install to an USB drive.
You can install the rEFInd boot manager if you wish, but consider this. The firmware on the Mac logic board contains a boot loader and boot manager. Ubuntu will by default install Grub, which is also a boot loader and boot manager. (Although in this case, the boot manager will operate silently.) Installing a rEFInd as a third boot manager is not necessary.