SpecialPreferences: Remove invalid <strong> tag in successbox

Using CSS font-weight instead.

Also removed all traces of #mw-pref-clear. They are redundant since the
preferences form already has class "visualClear".

Bug: 33438
Change-Id: If606203218ece5974a956a3d41d5db40f55b2d5c
Merge "SpecialPreferences: Remove invalid <strong> tag in successbox"