docs: arm: Remove orphan sh-mobile directory

This directory is empty, except for a .gitignore file, listing an
executable file that can no longer be built since commit
c6535e1e ("Documentation: Remove ZBOOT MMC/SDHI utility and
docs").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>