Remove non-existent command from usage text

The package manager used to support the 'dump enabled-overlays' command.
It was removed in commit 560830c9 "Track both framework and app
overlays per package".

Remove mention of the removed command from the usage text; this should
have been part of 560830c9 but was overlooked.

Test: "adb exec-out cmd package dump -h | grep -c -e 'enabled-overlays'" should print 0
Change-Id: I82028025b7271b855a0e8849c61231d00134b93e