Removed 4 unnecessary stubbings in OidcClientTest.java

[ARUS2023]

In our analysis of the project, we observed that

1. 3 unnecessary stubbings which stubbed getUserInfo, getTokenResponse(returned tokenResponse) and getUserInfoResponse are created but never executed by 2 tests: OidcClientTest.userInfoErrorResponse, OidcClientTest.userInfoErrorResponseWithoutErrorCode;

2. 1 unnecessary stubbing which stubbed getTokenResponse (returned invalidTokenResponse) is created but never executed by 1 test: OidcClientTest.getUserInfoFromUserInfoEndpoint;

Unnecessary stubbings are stubbed method calls that were never realized during test execution. Mockito recommends to remove unnecessary stubbings (https://www.javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/exceptions/misusing/UnnecessaryStubbingException.html).

We propose below a solution to remove the unnecessary stubbings.