Remove unnecessary alt-texts from pipeline emails

Open username-removed-600751 requested to merge gernberg/gitlab-ce:issue-36484 into master

What does this MR do?

Remove alt-text from IMG-tags in pipeline emails that doesn't benefit / need any alt texts. In accordance with: https://www.w3.org/WAI/tutorials/images/decorative/

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

When images aren't loaded in html emails it looked broken

Screenshots (if relevant)

Before

Screen_Shot_2017-09-30_at_17.48.59

After

Screen_Shot_2017-09-30_at_17.39.44

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

#36484

Edited by username-removed-600751