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>