integrator: remove empty ap_init_early()

The ap_init_early function is defined a global but has no declaration,
so it produces a warning:

arch/arm/mach-versatile/integrator_ap.c:148:13: warning: no previous prototype for 'ap_init_early' [-Wmissing-prototypes

The function could be made 'static' but since it's empty, we can
just remove it.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>