Removed numeric and mercury comparators, further refactoring

[andrzejj0]

Third stage of the refactoring: removed the Numeric and Mercury comparators, some further refactoring in DefaultVersionsHelper.

Because we're removing comparators, a new version of rules.xsd is necessary and it's a breaking change.

[slawekjaranowski]

We also should preserve old schema at site:
https://www.mojohaus.org/versions/versions-model/xsd/index.html
can be done at separate PR

[andrzejj0]

@slawekjaranowski would that be ok?

Versions - XML schemas for ruleSet
==================================

- [rule-3.0.0.xsd](rule-3.0.0.xsd)
- [rule-2.1.0.xsd](https://www.mojohaus.org/versions-maven-plugin/xsd/rule-2.1.0.xsd)
- [rule-2.0.0.xsd](https://www.mojohaus.org/versions-maven-plugin/xsd/rule-2.0.0.xsd)

[slawekjaranowski]

@slawekjaranowski would that be ok?

Versions - XML schemas for ruleSet
==================================

- [rule-3.0.0.xsd](rule-3.0.0.xsd)
- [rule-2.1.0.xsd](https://www.mojohaus.org/versions-maven-plugin/xsd/rule-2.1.0.xsd)
- [rule-2.0.0.xsd](https://www.mojohaus.org/versions-maven-plugin/xsd/rule-2.0.0.xsd)

try to generate site locally ...

we need an old link to works - https://www.mojohaus.org/versions/versions-model/xsd/rule-2.1.0.xsd
so we need a copy rule-2.1.0.xsd to versions-model/src/site/resources/xsd

[andrzejj0]

I will do the proper site in another PR...