Deprecate unused BUSYBOX_URL variable

[alexhaydock]

As of the most recent commit, 0de4bffe24c71604a19d95222011d1b2e6d41f10, I can see that the URL to download Busybox into the RHEL 8 UBI container is being hard-coded into the install.sh script:

https://github.com/splunk/docker-splunk/blob/0de4bffe24c71604a19d95222011d1b2e6d41f10/base/redhat-8/install.sh#L45

This seems to deprecate the previous lines: https://github.com/splunk/docker-splunk/blob/07a7c86f15abd13a1ee97661bdf8e4e762f8d1bf/base/redhat-8/install.sh#L81-L83

It seems like this was mainly being used in the Makefile for the armv8 RHEL8 container, passed in as a build-arg, which overrode the default: https://github.com/splunk/docker-splunk/blob/0de4bffe24c71604a19d95222011d1b2e6d41f10/Makefile#L77-L78

I guess this needed overriding for the armv8 container since Busybox don't seem to publish aarch64 builds at that URL above.

But now, as of 0de4bffe24c71604a19d95222011d1b2e6d41f10, we're building from source and not actually using the $BUSYBOX_URL variable anywhere, this is having no effect even if we do pass it in as a build-arg. So for clarity, we might as well remove the variable entirely.