structure: remove placeholder directories

Remove all placeholder directories, including build, scripts and src.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I5a7ccfe9540251473387ed17d376af0c34aff5ed