Don't bundle the Statistics Application by default

See http://markmail.org/message/f6fxnmrkobvylspz

Main rationale:

• Make default XWiki distribution smaller.
• Goes in the direction of making a minimal platform distribution
• Less visual clutter by default when running XWiki for the first time by an Admin
• The stats application needs some work to polish it and add features so it's better to make it visible only to people who only need it