samples: flash_shell: remove sample code and use FLASH_SHELL

The flash sample and flash shell have implementations that differ
providing similar features.
This commit removes all the code in main.c, only `main()` remains to
reduce redundancy.

Features or improved implementations has not been merged into the flash
shell in this commit.
It should be possible to do in future commits if desired.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>