Now that the nitro meter no longer features visible marks, adjust back the max nitro amount to what it was before, and restore back the old less easy nitro challenge on XR591
git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@14320 178a84e3-b1eb-0310-8ba1-8eac791a3b58