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The `signed-off-by` rule does not correctly detect the `Signed-off-by:`
trailer if it's not the last trailer. Therefore, this rule has been
disabled until we can resolve this in the commitlint upstream.

Change-Id: I50ea29067528f3c1c25beeea5eb25134b25b2af2
Signed-off-by: Chris Kay <chris.kay@arm.com>