fix: remove custom retry loop

[daniel-sanche]

Removing the hand-written _commit_with_retry logic, in favor of built-in gapic retry implementation

According to the docstrings, this custom retry logic was put in place because the gapic layer originally didn't retry ServiceUnavalable errors by default, but that has changed over time.

Now this retry logic is just stacking an extra retry loop on top of the built-in one. Additionally, the custom retry loop doesn't have an exit condition, making it susceptible to infinite loops

At this point, it is safer to remove this custom logic, and rely on only the built-in gapic retry behavior

More detailed refactors of retries coming later, but this is a step in that direction

Fixes #850