Fix two table style issue

[furybean]

Fixed:

1. fix table column width bug in safari [Bug Report] table components, under Safari, when tbody can scroll, there will be a table header dislocation after the scroll bar appears #7833
2. hide shadow of fixed when scrollX is false [Bug Report] [el-table] el-table-column use fixed="right" #8172

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[Leopoldthecoder]

这个改动会造成不必要的水平滚动条（比如文档上「固定表头」的例子），前提是滚动条始终显示：

[furybean]

@Leopoldthecoder 现在应该比较干净了，只修了两个 issue 里面的问题。