Don't uncomment ipynb raw cells

Author: mwouts

src/jupytext/cell_reader.py

@@ -862,6 +862,7 @@ def find_cell_end(self, lines):
             if self.metadata.get("active") == "":
                 del self.metadata["active"]
             self.cell_type = "raw"
+            self.comment = ""
         else:
             self.cell_type = "code"