Cache pairlist in pairlist, not globally

closes #4797 closes #4689

freqtrade/plugins/pairlist/IPairList.py

@@ -73,7 +73,7 @@ class IPairList(LoggingMixin, ABC):
         """
         raise NotImplementedError()
 
-    def gen_pairlist(self, cached_pairlist: List[str], tickers: Dict) -> List[str]:
+    def gen_pairlist(self, tickers: Dict) -> List[str]:
         """
         Generate the pairlist.
 
@@ -84,7 +84,6 @@ class IPairList(LoggingMixin, ABC):
         it will raise the exception if a Pairlist Handler is used at the first
         position in the chain.
 
-        :param cached_pairlist: Previously generated pairlist (cached)
         :param tickers: Tickers (from exchange.get_tickers()). May be cached.
         :return: List of pairs
         """