fix(Image): can not cancel previewMask

[buqiyuan]

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fix #5524

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[tangjinzhou]

忘了说我的想法了：
我们不用支持吧，给他个方案就可以了 不是有 maskClassName 这个了吗，可以满足他了

[buqiyuan]

忘了说我的想法了： 我们不用支持吧，给他个方案就可以了 不是有 maskClassName 这个了吗，可以满足他了

😂自定义maskClassName然后再设置display: none;吗？但这样感觉可能就没有将previewMask设置为false那么符合用户的直观，并且我也体验了一下vc-image，

在上面这个逻辑中，previewMask的值为falsy的时候都不会渲染mask，由于我们的previewMask是slot所以始终为truthy。所以我觉得这样也符合vc-image原本的一个逻辑，previewMask也相对于maskClassName更语义化一些，并且做这个支持的成本也不高，我也顺便移除了一些多余的代码

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