ProjectExplorer: De-virtualize knowsAllBuildExectuables method on Project

Just use a setter instead.

Change-Id: Id5dff64e9a7fcdf64a245b71432318299a49d9f7
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.h

@@ -65,8 +65,6 @@ public:
 
     QStringList buildTargetTitles() const;
 
-    bool knowsAllBuildExecutables() const final;
-
     ProjectExplorer::Tasks projectIssues(const ProjectExplorer::Kit *k) const final;
 
     void runCMake();