Remove duplicate ruby snippets

The snippets/ruby.snippets file defines two snippets
that already exist in the vim bundle.  When you try
to use either `let` or `its`, vim must ask you which
of the identical snippets you wish to use.

Removing the duplicates means that your snippets are
actually faster than typing the expanded text out
manually.
Merge pull request #37 from mkenyon/fix-vim-ruby-snippet

Remove duplicate ruby snippets