netpoll: Remove unused EXPORT_SYMBOLs of netpoll_poll and netpoll_poll_dev

Unused symbols waste space.

Commit 0e34e931
"(netpoll: add generic support for bridge and bonding devices)"
added the symbol more than a year ago with the promise of "future use".

Because it is so far unused, remove it for now.
It can be easily readded if or when it actually needs to be used.

cc: WANG Cong <amwang@redhat.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>