Remove broken m2e integration from the Eclipse Plugin

[afdia]

As mentioned in eclipse-m2e/m2e-core#991 it seems that the Umlet Plugin m2e integration is broken.
see also https://stackoverflow.com/questions/75245927/eclipse-failure-during-a-maven-update

Another report is eclipse-m2e/m2e-core#1129 and the only fix is to remove the Umlet plugin completely which is really bad for us.

Unfortunately I'm not really experienced with eclipse plugin development and even less with m2e integration, but as far as I have tried to get things running again, it doesn't work with our old tycho version and updating it didn't work either.

If I fix the class MavenProjectChangedListener.java by changing MavenProjectChangedEvent[] to List, the tycho plugin fails to compile, but we cannot update tycho without switching to min java version 11 and even then it didn't work for me (probably due to the old mars repo which we still use)

I guess if we don't find a developer with Eclipse Plugin experience who can fix such issues in the future, we will have to reduce the Eclipse integration to the simpler one as it was before #351 (less API interaction = less breakage)

But the Eclipse integration portion is quite old and still uses eclipse mars repos (in the pom.xml from eclipse plugin), so at one point we would have to modernize it or maybe discontinue the Eclipse plugin at one point in the future

This issue removes the m2e integration to fix the problem. Also I don't want m2e integration in Umlet, because other than the Eclipse Plugin API (which has been backwards compatible for more than one decade) it seems to change more often which is a problem for us.