soc: renesas: Remove ARCH_R8A7796

All users of CONFIG_ARCH_R8A7796 have been updated to use
CONFIG_ARCH_R8A77960, so the former can be removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191211100510.7182-1-geert+renesas@glider.be