Remove `role=complementary` from Post Update form

[mercime]

A <form> already has an implicit ARIA role=form and the only other ARIA roles which may be added for a <form> are role=search or role=presentation, both of which are not applicable for the post update form.

[mercime]

In 11415:

Activity: Remove role=complementary from Post Update form.

The ARIA role=complementary is not a valid markup for a <form>.

Fixes #7437.