lib-storage: shared driver - Remove copying the child storage's class_flags

lib-storage: shared driver - Remove copying the child storage's class_flags

The shared storage root itself doesn't have any storage, so it shouldn't
need to copy the child storage's class_flags either.

This fixes a crash when shared namespace was created with mdbox storage, and
another mdbox namespace was created afterwards. This had
MAIL_STORAGE_CLASS_FLAG_UNIQUE_ROOT set, but its unique_root_dir is NULL,
which caused a crash in mail_storage_match_class().