chore(serializer): remove unused serializer_max_series_points_per_payload setting

[tobz]

What does this PR do?

As stated in the PR title.

Motivation

serializer_max_series_points_per_payload was originally added in #11496 in order to add limits related to the new v2 series API, where there previously were no limits. Hard to divine intent from a six-year old PR, written by an engineer who no longer works here, but conceptually we can imagine that the only reason to change this would be if we wanted to change the limits to improve intake processing efficiency somehow.

However, we would do this by changing the default and not asking every Datadog customer to change it... which strongly implies that this should simply be a constant, rather than a tunable.

This is further bolstered by the fact that based on our internal usage data, only a single Agent installation out in the wild has this set to something other than the default value.

Describe how you validated your changes

Existing unit tests.

Additional Notes

[datadog-datadog-prod-us1]

✨ Fix all issues with BitsAI

⚠️ Warnings

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