9p/net: Remove unneeded idr.h #include

The 9p net files don't use IDR or IDA functionalities. So there is no point
in including <linux/idr.h>.
Remove it.

Link: https://lkml.kernel.org/r/9e386018601d7e4a9e5d7da8fc3e9555ebb25c87.1669560387.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>