Remove common secret keys from toml test config

[lukaszcl]

This PR removes certain test secrets, such as the Chainlink image and Loki credentials. Network secrets will be addressed in a separate ticket, which will include the removal of BASE64_NETWORK_CONFIG and related code (https://smartcontract-it.atlassian.net/browse/TT-1533)

Secrets will now be managed through the test secrets (see https://github.com/smartcontractkit/chainlink-testing-framework/blob/main/config/README.md#test-secrets).