Remove "selective" contextmods.

Remove "selective" contextmods.

This reverts those parts of commit
70365334614c31a82e9a3860c9eb9334cdc2879a that have been responsible
for allowing context-mods to contain modifications restricted to
certain context types.  It also removes use of this feature from
context-mod-from-music which is used for harvesting music into context
modifications.

As a result, any overrides in music used inside of context mods are
interpreted without looking at their actually targeted contexts.  It
turned out that this "feature" offered more confusion than benefits.