documentation: Remove sectnumlevels

Asciidoctor documentation says sectnumlevels from 0-5 are accepted.
Since every chapter sets this value to 6, it is not being parsed and
does not do anything.

Reviewed by: aokblast
Fixes: bdf18a045832 ("handbook: Remove sectnumlevels")
Differential Revision: https://reviews.freebsd.org/D57646