fbcon: delete unneeded function fbcon_takeover

Now there is no place use fbcon_takeover, and
fbcon_takeover has huge duplication code with
do_fbcon_takeover, we can achieve fbcon_takeover's
function with do_fbcon_takeover easily, so we
can just delete it.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>