btrfs: remove unused variable from find_free_extent

The variable found_uncached_bg in find_free_extent is not used since commit
285ff5af
(Btrfs: remove the ideal caching code)

Signed-off-by: Valentina Giusti <valentina.giusti@microon.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>