maintenance: stop writing log.excludeDecoration

maintenance: stop writing log.excludeDecoration

This reverts commit 96eaffebbf3d0 (maintenance: set
log.excludeDecoration durin prefetch, 2021-01-19).

The previous change created a default decoration filter that does not
include refs/prefetch/, so this modification of the config is no longer
needed.

One issue that can happen from this point on is that users who ran the
prefetch task on previous versions of Git will still have a
log.excludeDecoration value and that will prevent the new default
decoration filter from being active. Thus, when we add the refs/bundle/
namespace as part of the bundle URI feature, those users will see
refs/bundle/ decorations.

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