CMake: Remove CMakeProject::buildTargetTitles

The only user can also get this information from the relevant
CMakeBuildConfiguration.

Change-Id: I80e176ef0a8bc427f6adbf75b20e29f38d7b949f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.h

@@ -71,6 +71,8 @@ public:
 
     Utils::CommandLine cmakeCommand(ProjectExplorer::RunConfiguration *rc) const;
 
+    QStringList knownBuildTargets();
+
     QVariantMap toMap() const override;
 
     static QString cleanTarget();