Instead of remove to remove association with defect clicked on Delete button . tried to restore but did not find option to restore in recycle bin
There isn't a way to restore the defect suites but Deleting a defect suite does not delete the defects. Only the association between the defect suite and defects are removed and since the defects are not deleted that is why you don't see them in the recycle bin
You will need to recreate the defect suite and re associate the defects.
Manage Defect Suites | CA Agile Central Help
Please let us know if you have an questions.