introduce a function deleting dd's element

- required for abrt-python Python API
- required for implementation of abrt's DeleteElement D-Bus method.
- trac#754

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Richard Marko <rmarko@fedoraproject.org>

Conflicts:
	src/include/dump_dir.h
	src/lib/dump_dir.c