Clean up code relating to decision_policy and forecast_policy.

[lizard-boy]

We'll need to ship a yelpsoa-configs change that stops setting forecast_policy before we ship this.

[lizard-boy]

@greptileai

[greptile-apps]

PR Summary

Based on the provided information, here's a summary of the key changes in this pull request:

1. Removal of 'forecast_policy' and 'decision_policy': These options have been removed from the autoscaling configuration across the codebase, including documentation, code, and tests.

2. Deletion of forecasting functionality: The 'paasta_tools/autoscaling/forecasting.py' file and its corresponding test file have been completely removed, indicating a significant change in the autoscaling approach.

3. Documentation updates: The autoscaling documentation has been updated to reflect the removal of these policies and provide more detailed information about metrics providers and decision policies.

4. Schema changes: The autoscaling schema has been modified to remove the 'forecast_policy' property, which will impact how autoscaling is configured.

5. Code robustness: The code has been updated to handle the absence of 'decision_policy' more gracefully, preventing potential KeyError exceptions.

It's important to note that a yelpsoa-configs change to stop setting 'forecast_policy' needs to be shipped before this PR to ensure compatibility with existing configurations.

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