SLS-3123: Remove old code that caused a crash when trying to retrieve secrets manager keys

[IvanTopolcic]

What does this PR do?

This PR fixes a crash caused by trying to access a malformed secrets manager key, or a key which you do not have permission to access.

This code doesn't actually directly fix the issue, but removes obsolete code that caused the crash. This code should have been removed earlier but was mistakenly left in after a refactor.

The removed code is entirely duplicated in the following code:

// Set secrets from the environment that are suffixed with
// KMS_ENCRYPTED or SECRET_ARN
setSecretsFromEnv(os.Environ())

which we call directly above the removed code. This code is responsible for decrypting KMS environment variables, as well as calling secrets manager. I've tested this code with both working and malformed KMS keys and Secret ARNs. All four cases work.

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