[Core] Stop using removed disk.num_blocks_cache_hits stat

Removed in libtorrent v2.0.0:
569d47c391

.gitignore

@@ -22,3 +22,5 @@ deluge/ui/web/js/extjs/ext-extensions*.js
 osx/app
 RELEASE-VERSION
 .venv*
+# used by setuptools to cache downloaded eggs
+/.eggs

deluge/core/core.py

@@ -358,8 +358,8 @@ class Core(component.Component):
 
         if blocks_read:
             self.session_status['read_hit_ratio'] = (
-                self.session_status['disk.num_blocks_cache_hits'] / blocks_read
-            )
+                blocks_read - self.session_status['disk.num_read_ops']
+            ) / blocks_read
         else:
             self.session_status['read_hit_ratio'] = 0.0