[FEATURE][processing] Remove duplicate fill holes algorithm

The Fill_holes script is duplicated by the DeleteHoles algorithm,
and the DeleteHoles algorithm has many advantages:
- it's a full algorithm rather than a script
- uses qgis geometry classes and maintains curves/z/m
- has unit tests
- doesn't rely on non-standard external library (shapely)

(marked as feature for inclusion in release notes)

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

@@ -41,7 +41,7 @@ class DeleteHoles(GeoAlgorithm):
     def defineCharacteristics(self):
         self.name, self.i18n_name = self.trAlgorithm('Delete holes')
         self.group, self.i18n_group = self.trAlgorithm('Vector geometry tools')
-        self.tags = self.tr('remove,delete,drop,holes,rings')
+        self.tags = self.tr('remove,delete,drop,holes,rings,fill')
 
         self.addParameter(ParameterVector(self.INPUT,
                                           self.tr('Input layer'), [dataobjects.TYPE_VECTOR_POLYGON]))