remove some PMD warnings

as discussed in yesterday's meeting I enabled pmd and findbugs results for Catroid-Multi-Job and Catroid-Multi-Job-Custom-Branch!
For now the build does NOT fail if one of the tools reports errors/warnings - it is used as a midterm code quality visualization. But we should aim for 0 warnings (after ignoring unnecessary rules)!!

this pullrequest deals with PMD only - we use a smaller subset of checks, where only two warnings are ignored: https://github.com/Catrobat/Jenkins/blob/master/configs/catroid_pmd.xml
currently 207 PMD warnings in master: https://jenkins.catrob.at/view/All-Categories/view/Util/job/Catroid-Code-Checker-master/49/
in this pullrequest I fixed several warnings concerning unused modifier and useless overrides (see commit messages for mor info) - warnings down to 123 :)

testrun: https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/778/