run_model: remove assigning MODEL to a default path

MODEL environment variable should be set first before invoking the
scripts. So remove assigning MODEL environment variable to a default
path if $MODEL is empty.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ib3b0f2a53d4b4b29a6fd650d8272b1f9fac3fbcd