HW Relnotes: Remove xrpu(4)

[ziaee]

Summary

The xrpu driver was allegedly removed in 2007. Remove it's mention
from the Release Hardware Notes template and the last four years of
releases.

[emaste]

Not allegedly -- it was indeed removed in 2007 in rG:d31fc8ce59a07401e3e8c2599987d335b956b826

[ziaee]

I'm not sure what I didn't do right, or if something's hurt in our cluster, but I committed this as https://cgit.freebsd.org/doc/commit/?id=f8006b944cae79a909bc9d647ede5f5d8b7d7175