x86/asm: Fix typo in arch/x86/kernel/asm_offset_64.c

Drop double entry for pt_regs_bx.

This seems to be a typo - resulting in a double entry in the
generated include/generated/asm-offsets.h, which is not necessary.

Build-tested and booted on x86 64 box to make sure it was not
doing any strange magic.... after all it was in the kernel in
this form for almost 10 years.

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20141027172805.GA19760@opentech.at
Signed-off-by: Ingo Molnar <mingo@kernel.org>