x86: Remove unused thread_return label from switch_to()

Remove unused thread_return label from switch_to() macro on
x86-64. Since this symbol cuts into schedule(), backtrace at the
latter half of schedule() was always shown as thread_return().

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
LKML-Reference: <20091105160359.5181.26225.stgit@harusame>
Signed-off-by: Ingo Molnar <mingo@elte.hu>