clean up code

[stutzmann72]

Description:

• First version was released. Before adding to the pkp plugin gallery some code has to be cleaned up. See https://forum.pkp.sfu.ca/t/iiif-viewer-plugin-for-ojs-omp-3-4-released/91135/2
• Best done by Peter West. But budget must be clarified before. See comments by Alec Smecher https://github.com/ub-unibe-ch/iiifViewer/commit/2c522984dc749e51c3307576a746e953a63f54bb .

Acceptance:

### Tasks
- [x] person(s) in charge defined
- [ ] code cleaned up