remove unused modifier

pmd reported unused modifier in these files!
interface methods are implicitly abstract and public.
interface attributes are implicitly public static final!!
remove useless overrides

pmd reported UselessOverridingMethods!
Hide and show the SelectAll button appropriately
Add SelectAll to FormulaEditorVariableListFragment
Merge pull request #806 from Catrobat/pmdCleanup

remove some PMD warnings
Merge pull request #776 from Catrobat/SelectAllButton

Hide and show the SelectAll button appropriately
Merge & conflict resolve