Boot Camp Assistant, BootROM firmware

I have a partition on an older MacBook Pro running OS X 10.5.8 which has Windows XP on the partition, installed through Boot Camp.

I want to remove the partition and the Windows XP.

When I try to start Boot Camp Assistant, it shows the following message:

How do I go about doing this?

Is Boot ROM firmware part of EFI firmware which I am seeing on the software update window?

Is there a way to erase the partition and then join it back with the original hard disc volume?