Administration: Fix alignment of Recent Comments in the Activity widget.

[itzmekhokan]

The #latest-comments container already has the .activity-block class which provides margin: 0 -12px 6px -12px and padding: 8px 12px 4px to extend the block edge-to-edge within #dashboard_activity .inside while keeping its inner content aligned with sibling activity blocks.

The additional margin: 0 -12px on #latest-comments #the-comment-list made the comment list and the comment-item backgrounds break out of the activity-block padding a second time, leaving the list horizontally misaligned with the sibling <h3> heading, the views (subsubsub) navigation, and the reply form output below it.

Removing the redundant negative margin lets the comment list respect the parent .activity-block padding, restoring consistent left/right alignment for all elements inside the Activity widget.

Fixes #65217.

Trac ticket: https://core.trac.wordpress.org/ticket/65217

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