scripts/kernel-doc: simplify function printing

Get rid of the $start variable, since it's really not necessary.

No functional change.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215134828.1277109-3-vegard.nossum@oracle.com