Remove discussion of loop variables in the form x[a] or x.a.

Deprecation of that form is a subject encompassing all for-loop constructs
and is not specific to this PEP.  Also, most people are not aware that those
forms are even possible, so it is prudent to not even bring it up as that
would inevitably increase the number of people trying it in their code.

pep-0289.txt

@@ -152,9 +152,6 @@ semantic and syntactic specification.)
        y = list(x for x in "abc")
        print x    # prints "hello", not "c"
 
-   (Loop variables may also use constructs like x[i] or x.a; this form
-   may be deprecated.)
-
 4. All free variable bindings are captured at the time this function
    is defined, and passed into it using default argument values.  For
    example::