[processing] fix duplicates fields name in field calculator algorithm when switching between layers (followup c2699b4)

Author: slarosa

python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py

@@ -116,10 +116,8 @@ def initContext(self):
 
     def updateLayer(self, layer):
         self.layer = layer
-
-        self.initContext()
         self.builder.setLayer(self.layer)
-        self.builder.loadFieldNames()
+        self.initContext()
         self.populateFields()
 
     def setupSpinboxes(self, index):