Remove unused test files

[flo-dup]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines

Does this PR already have an issue describing the problem?
No

What kind of change does this PR introduce?
Cleaning

What is the current behavior?
Many reference files are unused in powsybl-iidm-serde unit tests

What is the new behavior (if this is a feature change)?
The unused files are deleted.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[olperr1]

Do we really want to have no tests on previous versions?

I think the problem here is essentially the fact that only the last version is checked.
Shouldn't we have something like this instead?

testForAllVersionsSince(IidmVersion.V_1_0, v -> {
    try {
        testWriteVersionedXml(network, options.setTopologyLevel(TopologyLevel.NODE_BREAKER), "fictitiousSwitchRef.xml", v);
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
});