readme: remove ancient build notes

Remove the ancient and unmantained src/BUILDNOTES readme file.

Change-Id: I652fded2526dbe97f6f87ef7263a2b39038486d5
Reviewed-on: http://gerrit.openafs.org/10971
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>