[#2899] Remove redundant method declaration

- Re-declaring a method as abstract when defined in an interface being
  implemented can lead to issues under some versions of PHP. Removing
  the abstract method in the abstract class fixes the issue.
Merge commit 'refs/pull/3681/head' of github.com:zendframework/zf2 into hotfix/declare-abstract-method
Merge pull request #3681 branch 'hotfix/declare-abstract-method' Fix #2899