Removed dead links in style-guide (#2952)

Author: Jinjiang

src/v2/style-guide/index.md

@@ -1790,7 +1790,7 @@ Prefer class selectors over element selectors in `scoped` styles, because large
 
 To scope styles, Vue adds a unique attribute to component elements, such as `data-v-f3f3eg9`. Then selectors are modified so that only matching elements with this attribute are selected (e.g. `button[data-v-f3f3eg9]`).
 
-The problem is that large numbers of [element-attribute selectors](http://stevesouders.com/efws/css-selectors/csscreate.php?n=1000&sel=a%5Bhref%5D&body=background%3A+%23CFD&ne=1000) (e.g. `button[data-v-f3f3eg9]`) will be considerably slower than [class-attribute selectors](http://stevesouders.com/efws/css-selectors/csscreate.php?n=1000&sel=.class%5Bhref%5D&body=background%3A+%23CFD&ne=1000) (e.g. `.btn-close[data-v-f3f3eg9]`), so class selectors should be preferred whenever possible.
+The problem is that large numbers of element-attribute selectors (e.g. `button[data-v-f3f3eg9]`) will be considerably slower than class-attribute selectors (e.g. `.btn-close[data-v-f3f3eg9]`), so class selectors should be preferred whenever possible.
 
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