vinyl: zap vy_env->indexes list

Before commit a63595db ("Implement index:info() introspection"),
we needed the list of all vinyl indexes to show index stats via
box.info.vinyl(). Now the list is only used in vy_env_delete() to
cleanup on shutdown. This code doesn't make much sense as it doesn't
delete VinylIndex structures - indexes should be deleted from the
upper level instead - so remove the cleanup and delete vy_env->indexes
as its meaning is quite obscure (e.g. it isn't quite clear when we
should add an index to it, from vy_index_new() or from vy_index_open()).