Remove rdoc data from dist in favor of rdoc-data gem

We have been shipping stale rdoc data for years as part of the JRuby dist, and it's time to just ditch it. The rdoc-data gem has ri data for all major releases of Ruby (or will as of Monday, when Eric Hodel will update it for 1.8.7, 1.9.2, 1.9.3). We will encourage users to install the rdoc gem and rdoc-data gem to get their rdoc/ri fix, and will no longer ship ri/rdoc content as part of JRuby.

This bug tracks the change, for release notes, etc.