Bluetooth: Remove mgmt_set_service_cache

Instead of having an explicit service cache command we can make the mgmt
API simpler by implicitly enabling the cache when mgmt_read_info is
called for the first time and disabling it when mgmt_set_dev_class is
called.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>