fix: v-scroll directive event listener leak

[rocka]

There's a heavy event listener leak when using v-scroll directive.

The performance profile was captured in https://muse-ui.org/#/zh-CN/directives . When the page scrolls, there's just tons of event listeners been attached, but none of them got removed.

https://github.com/museui/muse-ui/blob/master/src/internal/directives/scroll.js#L38

I think the update function in this directive was totally unnecessary, according to Vue Custom Directive Guide.

To demonstrate this, there's a fiddle: https://jsfiddle.net/zgrxt69h/ . update was removed, and it still works fine.

[codecov-io]

Codecov Report

Merging #1365 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1365     +/-   ##
=========================================
- Coverage   30.18%   29.98%   -0.2%     
=========================================
  Files         220      220             
  Lines        3578     3578             
  Branches      749      749             
=========================================
- Hits         1080     1073      -7     
- Misses       1880     1885      +5     
- Partials      618      620      +2

Impacted Files	Coverage Δ
src/internal/directives/scroll.js	81.25% <ø> (ø)	⬆️
src/utils/dom.js	0% <0%> (-13.21%)	⬇️

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