Remove categories HTML comments from 19.1 release notes
What does this MR change and why?
The <!-- categories: ... --> HTML comment in the 19.1 release note item files was a 19.1-only requirement that duplicated the categories: frontmatter field.
Now that 19.1 has shipped, the category validation relies on the categories: frontmatter field instead of the comment. This MR removes the redundant comments from the item files in doc/releases/19/gitlab-19-1-released/.
For every file changed, the removed comment's category matched the categories: frontmatter field, so no metadata is lost. Four files in the directory had no comment and are unchanged.
This follows on from the template cleanup that removed the same 19.1-only comment step from the release note process documentation.
Editing these older notes can fail the docs-lint release-note-categories job when a category has since been renamed or removed. That case is now handled by the pipeline:skip-release-note-categories label added in the related MR below, so no code changes are needed here.