ARM: shmobile: r8a7740: Remove restart callback

Remove the restart handling hack from the r8a7740 generic multiplatform
case.

Restart on DT-based r8a7740 platforms is now handled through the
R-Mobile reset driver.

This reverts commit 1174c712 ("ARM: shmobile: r8a7740: Add
restart callback").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>