Remove unused sysctl node

PR #14953 removed vdev-level read cache but accidentally left this
sysctl node behind.

Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Seth Hoffert <seth.hoffert@gmail.com>
Closes #16493