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/wallet/dump.cpp

@@ -37,7 +37,7 @@ bool DumpWallet(const ArgsManager& args, WalletDatabase& db, bilingual_str& erro
         return false;
     }
     std::ofstream dump_file;
-    dump_file.open(path);
+    dump_file.open(path.std_path());
     if (dump_file.fail()) {
         error = strprintf(_("Unable to open %s for writing"), fs::PathToString(path));
         return false;