vim: <Leader>e uses unite mkbfiles

By using the latest unite integration as part of the vim-mkbuild plugin,
dmenu is no longer required.  Don't get me wrong -- dmenu is a stellar
tool and I use it for many purposes on my desktop configuration.
However, I'd prefer for my vim configuration to be less dependent on
desktop features.  Also, since I'm using unite in several other ways,
moving the method to select project source files to unite provides
greater consistency.