For continuity and follow-up reasons it is not possible to delete a worklog. Instead if it has been entered by mistake, has faulty data or needs to be removed for some other reason you Revoke it. It can be seen as a deactivation of the entry, the revoked worklog will not be invoiced.
When you Revoke worklog you must fill in the canceling form, stating revoke reason. After revoking worklog, it get status Revoked and new option Reason for Revoking will appear in dropdown action in table. Clicking this button displays popup with revoke reason.