socket: Remove unused kernel_sendmsg_locked

socket: Remove unused kernel_sendmsg_locked

The last use of kernel_sendmsg_locked() was removed in 2023 by
commit dc97391e6610 ("sock: Remove ->sendpage*() in favour of
sendmsg(MSG_SPLICE_PAGES)")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250112131318.63753-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>