datasets: remove useless NULL check

datasets: remove useless NULL check

Coverity did detect that the cleaning code is only reached with
Dataset *set being initialized so the check is useless.

** CID 1649392:       Null pointer dereferences  (REVERSE_INULL)
/src/datasets-context-json.c: 719           in DatajsonGet()
>>>     Null-checking "set" suggests that it may be null, but it has
already been dereferenced on all paths leading to the check.

** CID 1649391:       Null pointer dereferences  (REVERSE_INULL)
/src/datasets.c: 526           in DatasetGet()
>>>     Null-checking "set" suggests that it may be null, but it has
already been dereferenced on all paths leading to the check.