Remove unused builtins parameter in initializer

Unsupported parameter, see PyCQA/flake8@2d3e277
Causes unnecessary warnings

Author: ROpdebee

flake8_rst_docstrings.py

@@ -965,7 +965,7 @@ class reStructuredTextChecker(object):
 
     STDIN_NAMES = set(["stdin", "-", "(none)", None])
 
-    def __init__(self, tree, filename="(none)", builtins=None):
+    def __init__(self, tree, filename="(none)"):
         """Initialise."""
         self.tree = tree
         self.filename = filename