Cleanup and remove checks not directly related to makefs.
Remove the D_flag_cleanup function, as common_cleanup() for these
particular set of tests does two things:
1. Unmount the md(4) device.
2. Destroy the md(4) device.
Essentially, one should only call common_cleanup() if the test body
invokes mount_image(). This is not the case for D_flag_body().
Remove default atf_check flags for clarity.
No functional changes intended.