'Signed out successfully' message is not needed

Related to #24982 (closed) (Removing the 'Signed in successfully' message).

The 'Signed out successfully' message is redundant and not needed. It is clear when looking at an issue that you are signed in or not (from the fact that your avatar is in the upper right). When you sign out, you are also automatically taken to the 'sign in' page, which is another clear indicator that you aren't signed in.