tools/wltests: simplify code considering that $DEVICE is always defined
tools/wltests: simplify code considering that $DEVICE is always defined

Device is a mandatory parameter now, let's remove the cases for when it was
not defined.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

bjackman/lisa,credp/lisa,bjackman/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa