Remove matching guidelines from license-list-XML repo

[swinslow]

The matching guidelines are part of the SPDX spec (Annex C):

• https://spdx.github.io/spdx-spec/v3.0.1/annexes/license-matching-guidelines-and-templates/
• https://github.com/spdx/spdx-spec/blob/develop/docs/annexes/license-matching-guidelines-and-templates.md

I know we've gone back and forth on whether they should be kept in license-list-XML. I think that if they're staying in the spec itself, then we should only maintain them in spdx-spec, and remove them from license-list-XML (replacing them with a link to spdx-spec).

We should also compare the existing one in license-list-XML with what's in spdx-spec, to confirm on any changes that have happened and haven't been synced (if any).