Revert "Mark models/fixtures as generated (#24775)" (#24782)

They are not generated in some cases (although they can).

And we should keep an eye on these files when reviewing.

When reviewing, files can be marked as "Viewed" and then it is
collapsed.

Author: wxiaoguang

.gitattributes

@@ -1,7 +1,6 @@
 * text=auto eol=lf
 *.tmpl linguist-language=Handlebars
 /assets/*.json linguist-generated
-/models/fixtures/** linguist-generated
 /public/img/svg/*.svg linguist-generated
 /public/vendor/** -text -eol linguist-vendored
 /templates/swagger/v1_json.tmpl linguist-generated