Delete logback.xml

Deleting (or renaming [https://github.com/jhipster/generator-jhipster/commit/7fe116aa2d512d88d85102cbf9614346e2e210de]) the existing file "jhipster-sample-app/src/test/resources/logback.xml" is not taken into account when updating a project (yo jhipster). Anyway, it is identical to "jhipster-sample-app/src/test/resources/logback-test.xml".