ARM: shmobile: r8a7791: Remove legacy SMP fallback code

As of commit f89a5170

 ("ARM: shmobile: r8a7791: Prioritize DT
APMU support") in v4.8, non-DT enablement of SMP was left as a fallback
for backwards compatibility with old DTBs.

The time has come to drop backwards compatibility, hence remove the
fallback code.

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