[ticket/12886] Remove obsolete check for f_user_lock

The user shouldn't be able to unlock his own topic if he doesn't have m_lock
permissions. Also, the check for f_user_lock was never going to evaluate to
true due to the topic not being unlocked making it obsolete anyway.

PHPBB3-12886
Merge pull request #2822 from marc1706/ticket/12886

[ticket/12886] Remove obsolete check for f_user_lock

* marc1706/ticket/12886:
  [ticket/12886] Remove obsolete check for f_user_lock