Bluetooth: Mesh: Remove outdated RPL entries in settings work item

This allows to move flash operations from the context that triggered
IVI update to the mesh settings work item that runs on the system
workqueue. After this change, the outdated RPL entries will be removed
in the setting work item triggered by store_pending function in
mesh/settings.c. This is required to for a case where the mesh settings
work item is running on a separate thread instead of the system
workqueue to unblock the system workqueue eventually.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>