Remove LinkButton from docs

LinkButton was removed in #1880

Author: siddharthkp

docs/src/@primer/gatsby-theme-doctocat/nav.yml

@@ -89,8 +89,6 @@
       url: /LabelGroup
     - title: Link
       url: /Link
-    - title: LinkButton
-      url: /LinkButton
     - title: Overlay
       url: /Overlay
     - title: Pagehead