Deleting the config.collections entry before the config.chunks entries ensures that a CatalogCache refresh won't see partial chunks.

In particular, if we don't do this we may trip this invariant . The CatalogCache refresh could see that the allowMigrations field in config.collections was changed, but the chunk that was bumped along with it may be already deleted by the dropCollection, so the CatalogCache doesn't see a new version.