Remove 2 unused Maven plugins

Cobertura can be used to calculate test coverage, but I don't think that
we are using it any more. I at least cannot find any usage of this
plugin.
Docs can be found here:
https://www.mojohaus.org/cobertura-maven-plugin/

Build Helper Maven Plugin provides several build utilities, but I also
can't find any place where we actually use the plugin.
Docs: https://www.mojohaus.org/build-helper-maven-plugin/

Signed-off-by: Florian Hockmann <florian.hockmann@gdata.de>

Author: FlorianHockmann

pom.xml

@@ -493,24 +493,11 @@
                     <artifactId>maven-gpg-plugin</artifactId>
                     <version>${maven.gpg.version}</version>
                 </plugin>
-                <plugin>
-                    <groupId>org.codehaus.mojo</groupId>
-                    <artifactId>build-helper-maven-plugin</artifactId>
-                    <version>3.5.0</version>
-                </plugin>
                 <plugin>
                     <groupId>org.codehaus.mojo</groupId>
                     <artifactId>exec-maven-plugin</artifactId>
                     <version>3.2.0</version>
                 </plugin>
-                <plugin>
-                    <groupId>org.codehaus.mojo</groupId>
-                    <artifactId>cobertura-maven-plugin</artifactId>
-                    <version>2.7</version>
-                    <configuration>
-                        <aggregate>true</aggregate>
-                    </configuration>
-                </plugin>
                 <plugin>
                     <artifactId>maven-antrun-plugin</artifactId>
                     <version>3.1.0</version>