Remove superfluous _validate from Data node

The Data nodeclass overwrote the `_validate` function but
 (a) did no validation
 (b) included commented code that referenced a private git repository
 (c) returned `None` instead of `True`.

The outcommented code involved validation of the `source` attribute and
the referenced issue pointed out that the value of this attribute was
not officially standardized (potentially leading users to using it in
different ways).

Since there has not been validation of this field since 2015, it seems
safe to assume that it cannot be easily reintroduced at this point.

aiida/orm/nodes/data/data.py

@@ -25,7 +25,7 @@ class Data(Node):
 
     Architecture note:
     Calculation plugins are responsible for converting raw output data from simulation codes to Data nodes.
-    Data nodes are responsible for validating their content (see _validate method).
+    Nodes are responsible for validating their content (see _validate method).
     """
     _source_attributes = ['db_name', 'db_uri', 'uri', 'id', 'version', 'extras', 'source_md5', 'description', 'license']
 
@@ -351,23 +351,3 @@ def _get_converters(self):
         valid_format_names = [i[len(exporter_prefix):] for i in method_names if i.startswith(exporter_prefix)]
         valid_formats = {k: getattr(self, exporter_prefix + k) for k in valid_format_names}
         return valid_formats
-
-    def _validate(self):
-        """
-        Perform validation of the Data object.
-
-        .. note:: validation of data source checks license and requires
-            attribution to be provided in field 'description' of source in
-            the case of any CC-BY* license. If such requirement is too
-            strict, one can remove/comment it out.
-        """
-        # Validation of ``source`` is commented out due to Issue #9
-        # (https://bitbucket.org/epfl_theos/aiida_epfl/issues/9/)
-        # super()._validate()
-        # if self.source is not None and \
-        #    self.source.get('license', None) and \
-        #    self.source['license'].startswith('CC-BY') and \
-        #    self.source.get('description', None) is None:
-        #     raise ValidationError("License of the object ({}) requires "
-        #                           "attribution, while none is given in the "
-        #                           "description".format(self.source['license']))

aiida/orm/nodes/node.py

@@ -175,13 +175,15 @@ def initialize(self) -> None:
         self._incoming_cache = list()
 
     def _validate(self) -> bool:
-        """Check if the attributes and files retrieved from the database are valid.
+        """Validate information stored in Node object.
 
-        Must be able to work even before storing: therefore, use the `get_attr` and similar methods that automatically
-        read either from the DB or from the internal attribute cache.
+        For the :py:class:`~aiida.orm.Node` base class, this check is always valid.
+        Subclasses can override this method to perform additional checks
+        and should usually call ``super()._validate()``!
 
-        For the base class, this is always valid. Subclasses will reimplement this.
-        In the subclass, always call the super()._validate() method first!
+        This method is called automatically before storing the node in the DB.
+        Therefore, use :py:meth:`~aiida.orm.entities.Entity.get_attribute()` and similar methods that automatically
+        read either from the DB or from the internal attribute cache.
         """
         # pylint: disable=no-self-use
         return True