It looks like you're running into an existing bug, which can be found here:
The workaround, in this case, is to manually delete the files:
1. Schedule downtime
2. Backup Jira
3. Stop Jira
4. Navigate to JIRA_HOME/plugins/installed-plugins directory
5. Delete/Remove out all jira-servicedesk*.jar or servicedesk* files, depending on what application you're looking to uninstall.
6. Start JIRA
Please be sure to watch it, so that you are notified when this is fixed in a future release.
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