AI 143430: Remove broken and unused code.

  This CL removes RoManager::getDevicePrivateKey() and its declaration.
  The method is not used anywhere and it contains bad code that returns
  reference to a local object.
  This is tested by building the master tree and running the framework
  testsuite.  No error was found in testsuite.
  BUG=1737538

Automated import of CL 143430