samples/littlefs: Fix rcar boards selecting Flash API

The rcar boards use block variant of LittleFS and Disk Access API,
but have been using proj.conf for Flash API based access.
The commit adds section building boards with pro_blk.conf
into sample.yaml, where rcar boards will now be allowed,
and removes redundant configuration lines from board specific
conf files.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>