net: lwm2m: remove data storage for IPSO Timer application type

We are pre-allocating a storage variable for the application type
resource in the IPSO Timer object.  This is an optional resource
which won't always be set by samples.

Let's leave out the pre-allocated variable and let the sample set
this reference if needed (it's optional).

Signed-off-by: Michael Scott <mike@foundries.io>