buildroot

fs: don't remove intermediate files

Now that they are in their own directory and no longer pollute the build
dir, there is no point in removing them.

Furthermore, a follow-up patch will require that those files survive
when more than one filesystem image is generated.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>