i18n: Avoid using HTML tags in translation strings (wp-admin/includes/meta-boxes.php)

[ramiy]

This patch removes HTML <b> tags from translation strings.

Replacing: Uploaded on: <b>%1$s</b>

With: Uploaded on: %s

The patch also rewrites the code abandoning the $datef and $stamp parameters. They are only used once. We don't need to declare them, we can do the same without declaring those params.

[SergeyBiryukov]

In 41910:

I18N: Remove <b> tag from a translatable string in post_submit_meta_box().

Props ramiy.
Fixes #41681.