[processing][needs-docs] Hide variable distance buffer from toolbox

This algorithm has become a model-only algorithm, since it's
functionality is now present in the native c++ buffer algorithm
(which can now buffer by field or expression).

Ideally we'd remove this algorithm entirely, but data defined
parameters are not yet supported by the modeler gui, so we need
to keep it around for use in models for now.

python/plugins/processing/algs/qgis/VariableDistanceBuffer.py

@@ -31,6 +31,7 @@
 
 from qgis.core import (QgsWkbTypes,
                        QgsProcessing,
+                       QgsProcessingAlgorithm,
                        QgsProcessingParameterFeatureSource,
                        QgsProcessingParameterField,
                        QgsProcessingParameterNumber,
@@ -67,6 +68,9 @@ def groupId(self):
     def __init__(self):
         super().__init__()
 
+    def flags(self):
+        return QgsProcessingAlgorithm.FlagSupportsBatch | QgsProcessingAlgorithm.FlagCanCancel | QgsProcessingAlgorithm.FlagHideFromToolbox
+
     def initAlgorithm(self, config=None):
         self.addParameter(QgsProcessingParameterFeatureSource(self.INPUT,
                                                               self.tr('Input layer')))