Remove plpython casts C source code casts:

Remove plpython casts C source code casts:

It removes last remaining casts inside struct definitions.
Such usage is bad practice, as it hides problems from compiler.

Reason for the cast is popular practice in some circles
to define functions as foo(MyObj *) instead of foo(PyObject *)
thus avoiding a local variable inside functions and make
direct calling easier.  As pl/python does not use such style,
the casts were unnecessary from the start.

Marko Kreen