rds: Remove delays and ContinuousTargetOccurence from DB instance operations

[berimbolo13]

Closes #44925

Removes hardcoded delays and continuous target occurrence checks before polling instance state during creation, stop, and deletion operations. The provider will now begin polling immediately with a 10-second interval.

This significantly reduces wait times for:

• Fast AWS provisioning scenarios
• Local testing with LocalStack (lightweight AWS emulator)

Background:
These delays were introduced in 2014 without explanation (originally committed to terraform repo without a PR). When Blue/Green deployments were introduced, delays were increased further with no documented rationale.

The ContinuousTargetOccurence parameter was added in commit 4e1733e (Nov 20, 2024) with only "Add ContinuousTargetOccurence" as the commit message, providing no context for why multiple consecutive checks are needed.

This change removes both the initial delays and the continuous target checks, allowing the provider to begin polling immediately whilst keeping the 10-second poll interval to prevent RDS API rate limiting.

Related:

• [Enhancement]: Adjust timings of resource deployments for local testing (e.g., with LocalStack) #28523 - LocalStack timeout discussion
• [Bug]: opensearch module waits 10 minutes for every update #32189 - OpenSearch update delay issue
• opensearch update delay: 10 min -> 1 min #32209 - OpenSearch update delay fix
• opensearch: Remove 10-minute delays from domain creation and deletion #45245 - OpenSearch creation/deletion delay fix

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