Fixes rendering duplicate data-src attribute

What does this MR do and why?

We are currently rendering on the issue list the assignee avatars with duplicate data-src attributes. (Only visible lookin at the source HTML as it is then replaced by JS). This also showed up in HTML errors as malformed HTML + duplicate information. Same base HAML template is also used on the Merge Request List.

<li class="gl-display-flex">
<a class="author-link has-tooltip" title="Assigned to Administrator" data-container="body" data-qa-selector="assignee_link" href="/root"><img width="16" class="avatar avatar-inline s16 lazy" alt="" data-src="/uploads/-/system/user/avatar/1/avatar.png?width=16" data-src="/uploads/-/system/user/avatar/1/avatar.png?width=16" src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" /></a>
</li>

After:

<li class="gl-display-flex">
<a class="author-link has-tooltip" title="Assigned to Administrator" data-container="body" data-qa-selector="assignee_link" href="/root"><img width="16" class="avatar avatar-inline s16 lazy" alt="" data-src="/uploads/-/system/user/avatar/1/avatar.png?width=16" src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" /></a>
</li>

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