Fix hyphen and underscore filename completion

This deletes the previous hack that allowed completing files with
the extension: e.g. `abcd.z` to `abcdefg.z`.

It is still possible to use `abcd[TAB].z`, and hyphens or underscores
are very much more important than this other trick.

Source:
https://github.com/robbyrussell/oh-my-zsh/issues/1398#issuecomment-169163149



Signed-off-by: Marc Cornellà <marc.cornella@live.com>