refactor: remove redundant class (#2143)

`rdflib.plugins.sparql.parserutils.plist` has no good purpose.

rdflib/plugins/sparql/parserutils.py

@@ -132,12 +132,6 @@ def __init__(self, name, expr):
         Param.__init__(self, name, expr, True)
 
 
-class plist(list):
-    """this is just a list, but we want our own type to check for"""
-
-    pass
-
-
 class CompValue(OrderedDict):
 
     """
@@ -244,7 +238,7 @@ def postParse(self, instring, loc, tokenList):
             if isinstance(t, ParamValue):
                 if t.isList:
                     if t.name not in res:
-                        res[t.name] = plist()
+                        res[t.name] = []
                     res[t.name].append(t.tokenList)
                 else:
                     res[t.name] = t.tokenList