[processing] Don't sort field names in multi-field input dialog

Field names should always be listed in the order that the layer
defines them

Author: nyalldawson

python/plugins/processing/gui/wrappers.py

@@ -1362,11 +1362,11 @@ def getFields(self):
         elif self.param.dataType() == QgsProcessingParameterField.DateTime:
             fieldTypes = [QVariant.Date, QVariant.Time, QVariant.DateTime]
 
-        fieldNames = set()
+        fieldNames = []
         for field in self._layer.fields():
             if not fieldTypes or field.type() in fieldTypes:
-                fieldNames.add(str(field.name()))
-        return sorted(list(fieldNames), key=cmp_to_key(locale.strcoll))
+                fieldNames.append(str(field.name()))
+        return fieldNames
 
     def setValue(self, value):
         if self.dialogType in (DIALOG_STANDARD, DIALOG_BATCH):