Remove unused includes (#3795)

* Remove unused include in Variant.h

* Remove unused include in util.h

* Remove unused include directives in util.cpp

* Remove unused include directives in TF2.h

* Remove unused include directives in TF2.cpp

* Remove unused include directive in TDB2.h

* Remove unused include directives in TDB2.cpp

* Remove unused include directives in Task.h

* Remove unused include directive in rules.cpp

* Remove unused include directives in rules.cpp

* Remove unused include directives in nag.cpp

* Remove unused include directive in main.h

* Remove unused include directive in legacy.cpph

* Remove unused include directive in Hooks.cpp

* Remove unused include directive in Filter.h

* Remove unused include directive in Filter.cpp

* Remove unused include directive in feedback.cpp

* Remove unused include directive in Eval.cpp

* Remove unused include directives in dependency.cpp

* Remove unused include directivess in Context.cpp

src/nag.cpp

@@ -25,15 +25,9 @@
 ////////////////////////////////////////////////////////////////////////////////
 
 #include <cmake.h>
-
-#include <algorithm>
 // cmake.h include header must come first
-
 #include <Context.h>
 
-#include <iterator>
-#include <unordered_set>
-#include <utility>
 #include <vector>
 
 ////////////////////////////////////////////////////////////////////////////////