The web route to delete action runners did not check if the ID that was
given belonged to the context it was requested in, this made it possible
to delete every existing runner of a instance by a authenticated user.
The code was reworked to ensure that the caller of the delete
runner function retrieved the runner by ID and then checks if it belongs
to the context it was requested in, although this is not an optimal
solution it is consistent with the context checking of other code for
runners.
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