Remove the restart handling hack from the sh73a0 generic multiplatform
case.

Restart on DT-based sh73a0 platforms is now handled through the
R-Mobile reset driver.

This basically reverts commit cad90081 ("ARM: shmobile: sh73a0:
Add restart callback").
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>