refactor: get wallet path relative to wallet_dir

Now that boost has been updated > 1.60, we can simplify how we get
wallet path relative to wallet_dir by using:
`boost::filesystem::lexically_relative`

src/wallet/db.cpp

@@ -12,7 +12,6 @@
 
 std::vector<fs::path> ListDatabases(const fs::path& wallet_dir)
 {
-    const size_t offset = wallet_dir.native().size() + (wallet_dir == wallet_dir.root_name() ? 0 : 1);
     std::vector<fs::path> paths;
     boost::system::error_code ec;
 
@@ -28,10 +27,7 @@ std::vector<fs::path> ListDatabases(const fs::path& wallet_dir)
         }
 
         try {
-            // Get wallet path relative to walletdir by removing walletdir from the wallet path.
-            // This can be replaced by boost::filesystem::lexically_relative once boost is bumped to 1.60.
-            const auto path_str = it->path().native().substr(offset);
-            const fs::path path{path_str.begin(), path_str.end()};
+            const fs::path path{it->path().lexically_relative(wallet_dir)};
 
             if (it->status().type() == fs::directory_file &&
                 (IsBDBFile(BDBDataFile(it->path())) || IsSQLiteFile(SQLiteDataFile(it->path())))) {