btrfs-progs: tests: remove misc/061-subvol-delete-qgroup test case

The test case relies on `--delete-qgroup` option which has been removed.

The feature needs to be implemented in kernel as it's more complex than
just calling an ioctl.  The test case does not take the complexity of
subvolume dropping into consideration and only tested the simplest
cases.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710