chore: delete unused C++ file

Author: Kha

src/library/CMakeLists.txt

@@ -3,7 +3,6 @@ add_library(library OBJECT expr_lt.cpp
   module.cpp dynlib.cpp replace_visitor.cpp num.cpp
   class.cpp util.cpp print.cpp annotation.cpp
   reducible.cpp init_module.cpp
-  projection.cpp
   aux_recursors.cpp
   profiling.cpp time_task.cpp
   formatter.cpp

src/library/projection.cpp

@@ -17,7 +17,6 @@ Author: Leonardo de Moura
 #include "library/suffixes.h"
 #include "library/annotation.h"
 #include "library/constants.h"
-#include "library/projection.h"
 #include "library/replace_visitor.h"
 #include "library/num.h"
 #include "githash.h" // NOLINT