Drop Melodic related tests
Since Melodic jobs were pruned in tecnalia_robotics/common_config!107, keeping related tests in this repository doesn't make sense (see failures to pull required images in pipelines associated to recent !101 (merged)).
Since Melodic jobs were pruned in tecnalia_robotics/common_config!107, keeping related tests in this repository doesn't make sense (see failures to pull required images in pipelines associated to recent !101 (merged)).