Stop Documentation/ly-examples/orchestra.ly from redefining markup commands

Stop Documentation/ly-examples/orchestra.ly from redefining markup commands

This does not seem required.  This patch does _not_ address multiple
warnings because of nested VerticalAxisGroup grobs.  Removing the
warnings is possible by removing the Axis_group_engraver from the
MarkLine context definition, but this messes up the positions of the
tempo marks.

[Keith, 2013-06-22 20:33:17]

> Removing the warnings is possible by removing the 
> Axis_group_engraver from the MarkLine context definition, 
> but this messes up the positions of the tempo marks.

You could keep the positioning and remove the warnings with
   \override MetronomeMark #'Y-offset  = #'()

The default has the tempo mark try to use side-position-interface, which doesn't
seem to work with no staff symbol.

[Graham Percival, 2013-06-24 08:49:27]

LGTM.

FWIW, this piece was composed and typeset by one of our blind users.  It's
amazing that he creates tweaks, but without being able to see their effects it's
not surprising that some of those tweaks turn out to be unnecessary.