netpoll: remove __netpoll_cleanup from exported API

netpoll: remove __netpoll_cleanup from exported API

Since commit 97714695ef90 ("net: netconsole: Defer netpoll cleanup to
avoid lock release during list traversal"), netconsole no longer uses
__netpoll_cleanup(). With no remaining users, remove this function
from the exported netpoll API.

The function remains available internally within netpoll for use by
netpoll_cleanup().

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250613-rework-v3-1-0752bf2e6912@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>