MINOR: channel: Remove CF_READ_ACTIVITY

Thanks to previous changes, CF_READ_ACTIVITY flags can be removed.
Everywhere it was used, its value is now directly used
(CF_READ_EVENT|CF_READ_ERROR).
diff --git a/src/stconn.c b/src/stconn.c
index 1876686..24b04cc 100644
--- a/src/stconn.c
+++ b/src/stconn.c
@@ -1226,7 +1226,7 @@
 
 		task_queue(task);
 	}
-	if (ic->flags & CF_READ_ACTIVITY)
+	if (ic->flags & (CF_READ_EVENT|CF_READ_ERROR))
 		ic->flags &= ~CF_READ_DONTWAIT;
 }