ProjectExplorer: Clean up IOutputParser interface

- Remove unneeded/unused functions.
- De-virtualize where possible.

In particular, after untangling a number of self-referential
redirections, it became apparent that the outputAdded()
infrastructure was entirely unused.

Change-Id: I51e1beed008df2727b42494b087efa476342397e
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmakeprojectmanager/qmakestep.cpp

@@ -338,7 +338,7 @@ void QMakeStep::runNextCommand()
     case State::RUN_MAKE_QMAKE_ALL:
         {
             auto *parser = new GnuMakeParser;
-            parser->setWorkingDirectory(processParameters()->workingDirectory().toString());
+            parser->setWorkingDirectory(processParameters()->workingDirectory());
             setOutputParser(parser);
             m_nextState = State::POST_PROCESS;
             startOneCommand(m_makeCommand);