Use only the agent token for deregistration during anti-entropy
The previous behavior had the agent attempt to use the "service" token
(i.e. from the `token` field in a service definition file), and if that
was not set then it would use the agent token.
The previous behavior was problematic because, if the service token had
been deleted, the deregistration request would fail. The agent would
retry the deregistration during each anti-entropy sync, and the
situation would never resolve.
The new behavior is to only/always use the agent token for service and
check deregistration during anti-entropy. This approach is:
* Simpler: No fallback logic to try different tokens
* Faster (slightly): No time spent attempting the service token
* Correct: The agent token is able to deregister services on that
agent's node, because:
* node:write permissions allow deregistration of services/checks on
that node.
* The agent token must have node:write permission, or else the agent
is not be able to (de)register itself into the catalog
Co-authored-by: Vesa Hagström <weeezes@gmail.com>
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