[ticket/13189] Do not use confirm box for marking all notifications read

We already use a token for preventing CSRF when marking notifications read.
Making a user confirm the marking read action serves no real purpose. No
information will be lost by just marking the notifications read but it will
prevent users from always having to confirm this action.

PHPBB3-13189
Merge pull request #3049 from marc1706/ticket/13189

[ticket/13189] Do not use confirm box for marking all notifications read
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13189] Do not use confirm box for marking all notifications read