btrfs: delete pointless BUG_ON check on quota root in btrfs_qgroup_account_extent()

The BUG_ON is deep in the qgroup code where we can expect that it
exists. A NULL pointer would cause a crash.

It was added long ago in 550d7a2e ("btrfs: qgroup: Add new qgroup
calculation function btrfs_qgroup_account_extents()."). It maybe made
sense back then as the quota enable/disable state machine was not that
robust as it is nowadays, so we can just delete it.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -2861,8 +2861,6 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
 	if (nr_old_roots == 0 && nr_new_roots == 0)
 		goto out_free;
 
-	BUG_ON(!fs_info->quota_root);
-
 	trace_btrfs_qgroup_account_extent(fs_info, trans->transid, bytenr,
 					num_bytes, nr_old_roots, nr_new_roots);