samples: fs: littlefs: sdmmc: remove fstab and fixed partition

For littleFS, the fstab entries in DTS are ignored currently.
So remove the overlay entries for the same. Also fixed-partitions
can't be used for sdmmc-disk. So remove the same. We just need to
have sdmmc node with zephyr,sdmmc-disk compatible string to have
littleFS working on disk.

build littleFS sample as follows:-

west build -p always -b stm32h747i_disco_m7 \
samples/subsys/fs/littlefs/ -- \
-DOVERLAY_CONFIG=boards/stm32h747i_disco_m7.conf \
-DCONF_FILE=prj_blk.conf

Signed-off-by: Murali Karicheri <murali.karicheri@sandc.com>