Remove explicit normalize_string_quotes after docstring fixes

`visit_STRING` finishes by calling `visit_default`, which already
deals with calling `normalize_string_quotes` -- and correctly checks
for `self.normalize_strings` first, which this callsite does not.

src/black/__init__.py

@@ -2093,7 +2093,6 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                     rf"\1\\{quote_char}", docstring
                 )
             leaf.value = leaf.value[0:3] + docstring + leaf.value[-3:]
-            normalize_string_quotes(leaf)
 
         yield from self.visit_default(leaf)