Delete EvalState::addToSearchPath

This function is now trivial enough that it doesn't need to exist.

`EvalState` can still be initialized with a custom search path, but we
don't have a need to mutate the search path after it has been
constructed, and I don't see why we would need to in the future.

Fixes #8229

src/libexpr/eval.hh

@@ -341,8 +341,6 @@ public:
         std::shared_ptr<Store> buildStore = nullptr);
     ~EvalState();
 
-    void addToSearchPath(SearchPath::Elem && elem);
-
     SearchPath getSearchPath() { return searchPath; }
 
     /**