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/cmakeprojectmanager/servermodereader.cpp

@@ -73,8 +73,6 @@ const int MAX_PROGRESS = 1400;
 
 ServerModeReader::ServerModeReader()
 {
-    connect(&m_parser, &CMakeParser::addOutput,
-            this, [](const QString &m) { Core::MessageManager::write(m); });
     connect(&m_parser, &CMakeParser::addTask, this, [this](const Task &t) {
         Task editable(t);
         if (!editable.file.isEmpty()) {