arch/tile: remove bogus performance optimization

We were re-homing the initial task's kernel stack on the boot cpu,
but in fact it's better to let it stay globally homed, since that
task isn't bound to the boot cpu anyway.  This is more of a general
cleanup than an actual performance optimization, but it removes
code, which is a good thing. :-)

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>