You are two degrees of separation from your answer. On the GEO page you provided:
Citation(s) Warf MB, Shepherd BA, Johnson WE, Bass BL. Effects of ADARs on small RNA processing pathways in C. elegans. Genome Res 2012 Aug;22(8):1488-98. PMID: 22673872
When you click on the PMID link, it takes you to the PubMed entry for the paper, there you find a link to the whole article, where you learn they used the Illumina Small RNA Prep Kit v1.0 or v1.5, and Novoalign to trim 3' adaptors.
You may use BBDuk with the options tpe and tbo, it should trim adapters even without knowing its sequence.
edit: read this thread on using BBDuk for small RNA, it says it doesn't work well for them. However, Brian Bushnell - the author of BBTools - updates them like crazy, it may well be fixed by now.
Thank you so very much. I mixed your answers and could find my adapter sequence!
It is: ATCTCGTATGCCGTCTTCTGCTTG
But i dont know if i cut the whole adapter sequence or the sequence overrepresented in fastqc, that is part of adapter sequence.