chore(MessageBox): unused keyframe

[zhousg]

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🧪 Playground Preview: https://element-plus.run/?pr=14335
Please comment the example via this playground if needed.

[YunYouJun]

Yeah, it's unused.

Just for more information:

We use msgbox-fade-in reverse instead of msgbox-fade-out.

element-plus/packages/theme-chalk/src/message-box.scss

Line 232 in e2167dc

animation: msgbox-fade-in getCssVar('transition-duration') reverse;