This check does not really makes sense as a group was never needed for a test to be executed on Travis. The testsuites that always have been used on Travis automatically execute all tests within the PHPUnit/Unit Integration and System folder. A group was never needed. Maybe some developers used groups to execute tests but then always only a part of the tests was executed as a test does not necessarily has the "correct" group. Removing this check is also important for plugin developers who write tests for their plugin as we cannot expect that they set a group as it is usually not needed in any other project. If plugin developers do not set a group and we run our automated tests no test would be executed because of this unneeded check.
Does it mean
@group Core is not needed on tests in Core?
Exactly. It is nice to have but not really mandatory and not even so useful. At some point we could maybe even add groups "dynamically" as we now core tests are always in
tests/PHPUnit/ and plugin tests are always in