Remove usage of libmozevent.utils.retry

[La0]

There is one remaining usage of retry in the events projects: it should be either

1. completely removed to rely only on Taskcluster retries,
2. or re-implemented with tenacity

[junngo]

Should It change libmozevent.utils.retry to tenacity decorator (i.e, @tenacity.retry())?

[marco-c]

As @La0 said, either use the built-in Taskcluster maxRetries or use tenacity.
I guess the first would be preferable so we don't have to add tenacity to the requirements.

In practice, retrying is already enabled by default by the taskcluster library, so I think we can just remove the usage of retry altogether and we'll be good.