btrfs: qgroup: remove pointless error check for add_qgroup_rb() call

The add_qgroup_rb() function never returns an error pointer anymore since
commit 8d54518b ("btrfs: qgroup: pre-allocate btrfs_qgroup to reduce
GFP_ATOMIC usage"), so checking for an error pointer result at
btrfs_quota_enable() is pointless.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>