command-list.txt: remove 'sparse-index' from main help

command-list.txt: remove 'sparse-index' from main help

Ever since 'git sparse-checkout' was introduced [1] it is included in
'git --help' in the section "work on the current change" along with
the commands 'add', 'mv', 'restore', and 'rm'.  It clearly doesn't
belong to that group, moreover it can't be considered such a common
command to belong to 'git --help' in the first place, so remove it
from there.

[1] 94c0956b60 (sparse-checkout: create builtin with 'list'
                subcommand, 2019-11-21)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>