Simplify fs::path by dropping filename() and make_preferred() overloads

These overloads were needed to allow passing `fs::path` objects directly to
libstdc++'s `fstream` constructors, but after the previous commit, there is no
longer any remaining code that does pass `fs::path` objects to `fstream`
constructors. Writing new code which does this is also discouraged because the
standard has been updated in https://wg21.link/lwg3430 to disallow it.

Dropping these also means its no longer possible to pass `fs::path` arguments
directly to `fstream::open` in libstdc++, which is somewhat unfortunate but not
a big loss because it is already not possible to pass them to the constructor.
So this commit updates `fstream::open` calls.

Additionally, this change required updates to src/bitcoin.cpp since it was
relying on the overloaded filename() method.

src/test/settings_tests.cpp

@@ -45,7 +45,7 @@ inline std::ostream& operator<<(std::ostream& os, const std::pair<std::string, c
 inline void WriteText(const fs::path& path, const std::string& text)
 {
     std::ofstream file;
-    file.open(path);
+    file.open(path.std_path());
     file << text;
 }