MIPS: SGI-IP27: Delete an unnecessary check before kfree() in hub_domain_free()

It can be known that the function “kfree” performs a null pointer check
for its input parameter.
It is therefore not needed to repeat such a check before its call.

Thus remove a redundant pointer check.

The source code was transformed by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>