instruments: fix midi by removing \type "Engraver_group"

David Kastrup explained:
> You explicitly override the setting inherited from \Staff with
>
> \type "Engraver_group"
>
> telling LilyPond to wire up this context for use with engravers, making
> it see and distribute grobs.  Which is not particularly helpful with
> \midi output.
instruments: remove unneeded \type "Engraver_group"

David Kastrup explained:
> Specifying \type rarely makes sense when not defining a context from
> scratch but inheriting from some existing definition.

See also http://code.google.com/p/lilypond/issues/detail?id=3535