Revert license key redaction code from PR 362

The redaction code in [PR 362](https://github.com/newrelic/go-agent/pull/362) has been superseded by the newer redaction code in [PR 415](https://github.com/newrelic/go-agent/pull/415). This means that license keys are now effectively redacted _twice_ under some circumstances.

This issue requests that the older one be removed, leaving the more generalized newer one in place.