[FTest] Remove retry library and use utils.py retryable decorator

[timgrein]

Related to #1566

This PR removes the retry library (only used once) as we're having exactly that functionality already implemented with our retryable decorator.

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[timgrein]

@retryable doesn't work for non-async functions. Will provide a PR for that functionality first.

Addressed in/depends on #1587.

[github-actions]

💔 Failed to create backport PR(s)

The backport operation could not be completed due to the following error:
There are no branches to backport to. Aborting.

The backport PRs will be merged automatically after passing CI.

To backport manually run:
backport --pr 1578 --autoMerge --autoMergeMethod squash