fix(ecs): add feature flag and remove ecs default deployment alarm settings

[GavinZZ]

Issue # (if applicable)

Internal ticket tracking V1142791950

Reason for this change

Originally in this PR #25840, we added default deployment alarm settings to fix an issue where adding deployment alarms, deploying your CFN stack, then removing the deployment alarms from the CFN template, and deploying again WILL NOT remove the deployment alarms from the service.

ECS now already supports default deployment alarm settings. We will remove the default setting of deploymentAlarms. Reason for removing this default behaviour is for region build where the deployment alarm service may not be available in new regions but is set by default by CDK.

Description of changes

Remove default deployment alarm.

Description of how you validated changes

All new tests and integration tests pass.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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