Revert "改善在较低硬件配置设备上欢迎界面卡顿的问题。"

This reverts commit b9d9a0b7200dae812fc8c98c9e2381e11585cad1.
Revert "增加判断摄像头设备是否存在来判断是否显示扫一扫"

This reverts commit 2b4b8852872f8ee509dd36d60e416a78a86ea646.