sparse-checkout: disable advice in 'disable'

sparse-checkout: disable advice in 'disable'

When running 'git sparse-checkout disable' with the sparse index
enabled, Git is expected to expand the index into a full index. However,
it currently outputs the advice message saying that that is unexpected
and likely due to an issue with the working directory.

Disable this advice message when in this code path. Establish a pattern
for doing a similar removal in the future.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>