remove osTicket (Unmaintained since Oct 2023)

- ref: awesome-selfhosted#1
- `ERROR:awesome_lint.py: osTicket: last updated -415 days, 1:22:42.573775 ago, older than 365 days`
- This is a well known project, but it seems like it is currently not maintained anymore. I found Issues asking about this as late as this September, but the response was that is still maintained (osTicket/osTicket#6746). However looking at the roadmap linked to, which was last updated in May it doesn't seem like there is any activity. So not too sure about this one..