Don't move to Staff context before voicifying music

Don't move to Staff context before voicifying music

This reverts a change introduced without explanation in commit
commit e4c229e52b6e102676b1bcae5aff89bf21401225
Author: Han-Wen Nienhuys <hanwen@xs4all.nl>
Date:   Fri Feb 6 23:49:41 2004 +0000

The symptom that this change was presumably supposed to address should
have been cured by commits

commit d7e9ab2f2782383025ffacd93a306f61f3ef7d45
Author: David Kastrup <dak@gnu.org>
Date:   Thu Sep 12 22:05:07 2013 +0200

    Issue 3553/2: When creating a hierarchy above Bottom, all intermediate levels are named "\new"
[...]

commit d756b5523a8d82491f734d79943314c5e6cd35b4
Author: David Kastrup <dak@gnu.org>
Date:   Thu Sep 12 19:25:17 2013 +0200

    Issue 3553/1: get_default_interpreter should create contexts with missing context_id
[...]

Not having to make assumptions about "Staff" makes the \\ have fewer
hidden semantics.