Investigate why we're failing to clean up SSM parameters

[cnunciato]

This morning we hit the limit of 10K parameters in the CI account:

An error occurred (ParameterLimitExceeded) when calling the PutParameter operation: You have reached the maximum number of standard parameters for this AWS account and Region (10000). To add more parameters, either delete existing parameters or upgrade to the advanced-parameter tier (charges apply).

We use these parameters to map commits to S3 buckets, and we should be deleting them with each bucket we delete, but for some reason this doesn't seem to be happening. I've deleted several thousand in the CI and production accounts, so we have some breathing room, but we should figure out why these aren't being cleaned up automatically.