Remove workaround for ambiguous between varargs and non-varargs

CC @njlbenn 
See details of workaround that needs to be removed as soon as Dotty issue [#9688](https://github.com/lampepfl/dotty/issues/9688) is resolved

See [comment](https://github.com/lunatech-labs/lunatech-scala-2-to-scala3-course/pull/67#discussion_r481853448) for details.

Note that this workaround is applied in 4 places in the code.