[Bug Report] Table clearSort() doesn't work on custom sort.

[Holybasil]

Element UI version

2.4.11

OS/Browsers version

71.0.3578.98

Vue version

2.5.21

Reproduction Link

https://jsfiddle.net/Holybasil/pqkn05s6/18/

Steps to reproduce

See jsfiddle and try to sort the two columns.

What is Expected?

clearSort() could have the consistent performance in default sort and custom sort.

What is actually happening?

clearSort() is not friendly to custom sort.

[ziyoung]

#15439