Remove unused includes list, functional from expr.h

This header file is widely used, and dragging in useless includes unnecessarily
slows down compilation.

Author: tautschnig

src/util/expr.h

@@ -14,9 +14,6 @@ Author: Daniel Kroening, [email protected]
 #include "validate_types.h"
 #include "validation_mode.h"
 
-#include <functional>
-#include <list>
-
 #define forall_operands(it, expr) \
   if((expr).has_operands()) /* NOLINT(readability/braces) */ \
     for(exprt::operandst::const_iterator it=(expr).operands().begin(), \