[processing] Remove extraneous parameter value check

The subsequent call to checkParameterValues covers the same checks,
but in a more flexible way.

Refs discussion at qgis/QGIS-Documentation#2797

(cherry-picked from 85d7400)

Author: nyalldawson

python/plugins/processing/core/Processing.py

@@ -124,14 +124,6 @@ def runAlgorithm(algOrName, parameters, onFinish=None, feedback=None, context=No
             feedback.reportError(msg)
             raise QgsProcessingException(msg)
 
-        # check for any mandatory parameters which were not specified
-        for param in alg.parameterDefinitions():
-            if param.name() not in parameters:
-                if not param.flags() & QgsProcessingParameterDefinition.FlagOptional:
-                    msg = Processing.tr('Error: Missing parameter value for parameter {0}.').format(param.name())
-                    feedback.reportError(msg)
-                    raise QgsProcessingException(msg)
-
         if context is None:
             context = dataobjects.createContext(feedback)