CMake: Remove BuildDirManager

The BuildDirManager was used to switch between different
BuildDirReaders. Now that only the FileApiReader is left, that
infrastructure is no longer needed.

Change-Id: I2d339a3407bb633cff6a8f7502b7b09094f63fef
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/cmakeprojectmanager/fileapireader.cpp

@@ -136,6 +136,8 @@ void FileApiReader::parse(bool forceCMakeRun, bool forceConfiguration)
 
 void FileApiReader::stop()
 {
+    if (m_cmakeProcess)
+        disconnect(m_cmakeProcess.get(), nullptr, this, nullptr);
     m_cmakeProcess.reset();
 }