util: json: Remove virJSONValueIsNull/virJSONValueObjectIsNull

util: json: Remove virJSONValueIsNull/virJSONValueObjectIsNull

If needed 'virJSONValueIsNull' can be easily replaced by
'virJSONValueGetType(obj) == VIR_JSON_TYPE_NULL'.

'virJSONValueObjectIsNull' has confusing name because it checks that a
virJSONValue of OBJECT type has a key which is NULL, not that the object
itself is NULL. This can be replaced according to the needs e.g. by
virJSONValueObjectHasKey or the above check.

Both are unused.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>