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/parser.y

@@ -736,12 +736,6 @@ Expr * EvalState::parseStdin()
 }
 
 
-void EvalState::addToSearchPath(SearchPath::Elem && elem)
-{
-    searchPath.elements.emplace_back(std::move(elem));
-}
-
-
 SourcePath EvalState::findFile(const std::string_view path)
 {
     return findFile(searchPath, path);