net: openthread: fix check for unsuccessful wipe

ot_setting_delete_subtree returns the information if the subtree
had been found in the persistent storage or not before removing.
This has matter for otPlatSettingsGet. In other uses cases
the return value shoud be ignored.
The right place for the check if deletion succeded is in
settings_load_subtree_direct and its callback.
Currently "ot factoryreset" causes an assert if there is no
OpenThread dataset stored in the persistent memory.
This PR fixes it.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>