Remove man page from pry

This removal was requested because the *nix man command does not find
it.  The pry man page (as it is currently) was used by a rarely used gem named gem-man, that
looked in a man folder for the correct page.

Until rubygems plans to load manpages into GEM_PATH for general use we'll remove this.

https://github.com/rubygems/rubygems/issues/74