doc: printk-formats: Remove bogus kobject references for device nodes

When converting from text to rst, the kobjects section and its sole
subsection about device tree nodes were coalesced into a single section,
yielding an inconsistent result.

Remove all references to kobjects, as
  1. Device tree object pointers are not compatible to kobject pointers
     (the former may embed the latter, though), and
  2. there are no printk formats defined for kobject types.

Update the vsprintf() source code comments to match the above.

Fixes: b3ed2321 ("doc: convert printk-formats.txt to rst")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>