Specify that PocketBook 515 has no frontlight (#6549)

It doesn't have one, and having it in there might lead to issues.

frontend/device/pocketbook/device.lua

@@ -216,6 +216,7 @@ local PocketBook515 = PocketBook:new{
     model = "PB515",
     display_dpi = 200,
     isTouchDevice = no,
+    hasFrontlight = no,
     hasDPad = yes,
     hasFewKeys = yes,
 }