Remove .katex-display from selector

Annabel Dunstone Grayrequested to merge
remove-unnecessary-css into master

I meant to fix this a while back. Originally added here - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8543

Seems that it's no longer needed:

header header header
Screen_Shot_2018-10-24_at_10.29.06_AM Screen_Shot_2018-10-24_at_10.29.17_AM Screen_Shot_2018-10-24_at_10.29.31_AM

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