doc: remove "maintenance is supported by" text in BUILDING.md

The "maintenance is supported by" stuff in BUILDING.md is unclear. It
seems unnecessary so I propose removing it.

I don't understand what it means to, in this context, support
maintenance. Does it mean that you simply do the maintenance? Does that
mean it really just means "maintain"? Do we really mean that we mantain
support, rather than support maintenance?

That information does not seem necessary to include. I'm not sure it's
meaningful. With (for example) Windows, is it accurate to say that the
Node.js core team maintains support for it? Or is it accurate to say
that support is maintaned by smaller groups or individuals within the
Node.js core team?  Both could be considered accurate. So is the
difference meaningful?

I think the more important elements of determinig tier support are the
other listed elements.

PR-URL: https://github.com/nodejs/node/pull/30365


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>