Revert r38876 "(bug 2443) Add image name as alt-text when no caption is provided."

Revert r38876 "(bug 2443) Add image name as alt-text when no caption is provided."
This broke four test cases:

   4 previously failing test(s) now PASSING! :)
      * Right-aligned image  [Fixed between 08-Aug-2008 21:37:38, 1.14alpha (r38954) and now]
      * Centre-aligned image  [Fixed between 08-Aug-2008 21:37:38, 1.14alpha (r38954) and now]
      * None-aligned image  [Fixed between 08-Aug-2008 21:37:38, 1.14alpha (r38954) and now]
      * Width + Height sized image (using px) (height is ignored)  [Fixed between 08-Aug-2008 21:37:38, 1.14alpha (r38954) and now]

Please recommit with fixes to the existing test cases and some new test cases to cover cases where an empty caption is explicitly requested, see https://bugzilla.wikimedia.org/show_bug.cgi?id=2443#c11