collapse_api: code cleanup

• Remove useless output buffering in collapse_api
  The obsolescence of $g_usejavascript setting (see commit
  75be0f4c15ec6aa2379c3a6504733bb28dc1b259) makes the various ob* calls
  unnecessary since the generated output is displayed in every case.
• phpDoc changes (revise references to javascript disabled)
• remove useless function call
• use a variable for collapse status class to improve readability