kernel/cpu.c: remove new instance of __cpuinit that crept back in

We removed __cpuinit support (leaving no-op stubs) quite some time ago.
However a new instance was added in commit 00df35f9
("cpu: Defer smpboot kthread unparking until CPU known to scheduler")

Since we want to clobber the stubs soon, get this removed now.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>