How to delete a label in Jira
To completely delete a label from a JIRA instance, the label must be first removed from all the issues in that instance. Once it is removed from all issues, it will no longer be seen when browsing a labels.
How do I bulk-remove a label?
If you are using JIRA Cloud applications, you can search for a label and perform a bulk edit operation (see Editing multiple issues) to remove it. Note that in JIRA Server applications, labels can only be replaced, rather than added or removed. See - JRA-24118Getting issue details... STATUS for progress on this feature.