Delete unused modes

[elharo]

We can always add these later if we need them, but right now their existence implies things about the code that aren't true.

[elharo]

some unrelated flakage in CI:

Error: Failed to read artifact descriptor for org.sonatype.sisu:sisu-guice:jar:no_aop:3.1.0: Failed to collect dependencies at org.apache.maven.plugins:maven-dependency-plugin:jar:3.3.0 -> org.apache.maven.shared:maven-common-artifact-filters:jar:3.2.0 -> org.eclipse.sisu:org.eclipse.sisu.plexus:jar:0.0.0.M5 -> org.sonatype.sisu:sisu-guice:jar:no_aop:3.1.0: The following artifacts could not be resolved: org.sonatype.sisu:sisu-guice:pom:3.1.0 (absent): Could not transfer artifact org.sonatype.sisu:sisu-guice:pom:3.1.0 from/to central (https://repo.maven.apache.org/maven2): HTTP connect timed out
Error: -> [Help 1]

[gnodet]

It's the default mode, so obviously not usually explicitly defined, though I could find a couple of code that does use it:
https://github.com/search?q=combine.self%3D%22merge%22+&type=code

Removing the default value would not allow specifying the default behaviour anymore, which may break some use cases (not 100% sure).

Also, this removes the only place where those values are documented and leave the code with no documentation at all.

In order to remove the fact they are not used, maybe the merge method could check if the attributes have known values (i.e. override or merge) and log something if that's not the case ?

But overall, I'm not sure what we are the benefits here, and I do see drawbacks.