ARM: shmobile: Remove unused hotplug.c

Each CPU Hotplug implementation for mach-shmobile
is now self-contained, so this change removes unused
helper code in hotplug.c. The two CPU Hotplug capable
SoCs sh73a0 and r8a7779 remain unchanged.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>