init: remove obsolete arch_call_rest_init() wrapper

Since commit 3570ee04 ("s390/smp: keep the original lowcore for
CPU 0"), there is no longer any architecture that needs to override
arch_call_rest_init().

Remove the weak wrapper around rest_init(), call rest_init() directly, and
make rest_init() static.

Link: https://lkml.kernel.org/r/aa10868bfb176eef4abb8bb4a710b85330792694.1706106183.git.geert@linux-m68k.org


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>