Simplify code of oscillator

[BenjaminRodenberg]

Removes temporary and unused variables.

It may help to have a look at the file CONTRIBUTING.md for a few hints and guidelines. -->

ToDo:

• Update oscillator-overlap after oscillator-overlap: Make consistent with oscillator #597 has been merged

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[BenjaminRodenberg]

@uekerman let's merge #597 first. I spotted some unused variables. Some other changes are maybe worth discussing. Would appreciate a second opinion on the changes if they really improve readability or not.

[uekerman]

Makes sense to me.

[NiklasVin]

The changes look good to me. I tested it with the runge_kutta-4 and radauIIA and they work fine