fix loading frontend/JSON.lua for json.lua on Kindle

The VFAT file system of `/mnt/us` is case insensitive so that
require("json") from Spore which intends to load luajson module in
rock directory actually will load `frontend/JSON.lua`, the two JSON
parser libraries have slightly different APIs.

This patch remove the pure Lua JSON library in favor of `luajson` which
uses LPeg pattern marching backend for better performance.

frontend/apps/reader/modules/readerwikipedia.lua

@@ -6,7 +6,6 @@ local Wikipedia = require("ui/wikipedia")
 local UIManager = require("ui/uimanager")
 local Geom = require("ui/geometry")
 local Screen = require("device").screen
-local JSON = require("JSON")
 local DEBUG = require("dbg")
 local _ = require("gettext")