x86/smp: Remove unnecessary initialization of thread_info::cpu

It's statically initialized to zero -- no need to dynamically
initialize it to zero as well.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/6cf6314dce3051371a913ee19d1b88e29c68c560.1468527351.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>