Some clean up on the PrometheusSink class

[dlvenable]

Description

I was looking at the behavior of the Prometheus sink for the AWS config and found that there were some unused code paths. This PR cleans them up. Additionally, this makes some improvements to using final and boolean primitives instead of objects.

It is actually used here:

data-prepper/data-prepper-plugins/prometheus-sink/src/main/java/org/opensearch/dataprepper/plugins/sink/prometheus/PrometheusSigV4Signer.java

Lines 50 to 60 in 63b2c43

	private static AwsCredentialsOptions convertToCredentialOptions(final AwsConfig awsConfig) {
	if (awsConfig == null) {
	return AwsCredentialsOptions.builder().build();
	}
	return AwsCredentialsOptions.builder()
	.withRegion(awsConfig.getAwsRegion())
	.withStsRoleArn(awsConfig.getAwsStsRoleArn())
	.withStsExternalId(awsConfig.getAwsStsExternalId())
	.withStsHeaderOverrides(awsConfig.getAwsStsHeaderOverrides())
	.build();
	}

Not in the code removed.

Issues Resolved

N/A

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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