[IA64] remove find_thread_for_addr()

find_thread_for_addr() is no longer needed.  It was only used to find
the correct kernel RBS for a given memory address, but since the kernel
RBS is not needed any longer, this function can go away.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Signed-off-by: Tony Luck <tony.luck@intel.com>