x86/realmode: Remove trampoline_status

There is no reader of trampoline_status, it's only written.

It turns out that after commit ce4b1b16 ("x86/smpboot: Initialize
secondary CPU only if master CPU will wait for it"), trampoline_status is
not needed any more.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1563266424-3472-1-git-send-email-kernelfans@gmail.com