Require use_sell_signal to be true for edge

Otherwise edge will have strange results, as
edge runs with sell signal, while the bot runs without sell signal,
causing results to be invalid

closes #3900

freqtrade/configuration/config_validation.py

@@ -137,6 +137,10 @@ def _validate_edge(conf: Dict[str, Any]) -> None:
             "Edge and VolumePairList are incompatible, "
             "Edge will override whatever pairs VolumePairlist selects."
         )
+    if not conf.get('ask_strategy', {}).get('use_sell_signal', True):
+        raise OperationalException(
+            "Edge requires `use_sell_signal` to be True, otherwise no sells will happen."
+        )
 
 
 def _validate_whitelist(conf: Dict[str, Any]) -> None: