Remove reference to afsinstall from src/README.md

Remove reference to afsinstall from src/README.md

Commit "deorbit afsinstall" (ed15b0d5b3) removed afsinstall from the
source tree, but not did remove where it was referenced in
src/README.md.

Remove the reference to 'afsinstall' from the 'Not Maintained' section
in src/README.md.

Change-Id: Ie5226bf97003c21044759bd5dc2b8efe5d9138c2
Reviewed-on: https://gerrit.openafs.org/14805
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>