lib: Drop IGT_EXIT_TIMEOUT

We use the timeout status for when the runner had to kill a testcase,
which indicates a more sever issue than an operation failing that we
expected to complete within seconds.

Since it's unused, drop it.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
13 jobs for master in 3 minutes and 10 seconds (queued for 1 second)
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  Deploy
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pages

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