gfs2: remove redundant set of INSTANTIATE_NEEDED

Function rgrp_go_inval calls gfs2_rgrp_brelse to invalidate the
in-core rgrp structures. After the call it set GLF_INSTANTIATE_NEEDED,
which is redundant, since gfs2_rgrp_brelse also sets it.
This patch simply removes the redundant set_bit.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>