Remove JWT validation from production code

[JLLeitschuh]

Motivation

CodeQL was flagging the current use of JWT as being vulnerable as validateBrokerToken wasn't actually performing validlidation of the signature.

Since the logic is unused, except for in test, the entire chunk of logic has been moved exclusively to tests.

Explain here the context, and why you're making that change. What is the problem you're trying to solve.

Modifications

Move JwtServiceImpl#validateBrokerToken logic into BrokerTokensServiceImplTest

Verifying this change

• [x] Make sure that the change passes the ./gradlew build checks.