sh: fix kexec by removing check for old kexec-tools

This unbreaks kexec support. Without this fix all
cases of kexec fails since __pa() does not behave
like PHYSADDR(). The downside is that we also kill
the code blocking users running old kexec-tools.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>