Revert "operand: Implement operand hashing and use hashed set for declarations"

This reverts commit 1e665afa36.

src/literal_string.cpp

@@ -4,7 +4,6 @@
  * 3-Clause BSD License
  */
 
-#include <functional>
 #include <string>
 #include "common_types.h"
 #include "literal_string.h"
@@ -37,8 +36,4 @@ bool LiteralString::operator==(const Operand& other) const {
     return false;
 }
 
-std::size_t LiteralString::Hash() const {
-    return Operand::Hash() ^ std::hash<std::string>{}(string);
-}
-
 } // namespace Sirit