Added tests and docstring to exchange funding_fee methods, removed utils

tests/exchange/test_exchange.py

@@ -2994,3 +2994,15 @@ def test_get_funding_fees(default_conf, mocker, exchange_name):
         pair="XRP/USDT",
         since=unix_time
     )
+
+
+def test_get_mark_price():
+    return
+
+
+def test_get_funding_fee_dates():
+    return
+
+
+def test_calculate_funding_fees():
+    return