When converting a table to a view, remove its system columns.

When converting a table to a view, remove its system columns.

Views should not have any pg_attribute entries for system columns.
However, we forgot to remove such entries when converting a table to a
view.  This could lead to crashes later on, if someone attempted to
reference such a column, as reported by Kohei KaiGai.

Patch in HEAD only.  This bug has been there forever, but in the back
branches we will have to defend against existing mis-converted views,
so it doesn't seem worthwhile to change the conversion code too.