cleanup of rack variables.

During a recent deep dive into all the variables so I could
discover why stack switching caused larger retransmits I examined
every variable in rack. In the process I found quite a few bits
that were not used and needed cleanup. This update pulls
out all the unused pieces from rack. Note there are *no* functional
changes here, just the removal of unused variables and a bit of
spacing clean up.

Reviewed by: Michael Tuexen, Richard Scheffenegger
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D34205