Clean the REMIND coupling interface

[dklein-pik]

🐦 Description of this PR 🐦

Clean the REMIND coupling interface (remove deletePlus, clean code)

🔧 Checklist for PR creator 🔧

• Label pull request from the label list.

• Self-review own code

• Document changes

• Perform test runs

📉 Performance changes 📈

• Current develop branch default : ** mins
• This PR's default : ** mins

🚨 Checklist for reviewer 🚨

• PR is labeled correctly
• Code changes look reasonable
  • No hard coded numbers and cluster/country/region names.
  • No unnecessary increase in module interfaces
  • model behavior/performance is satisfactory.
• Changes are properly documented
  • CHANGELOG is updated correctly
  • Updates in interfaces have been properly addressed in the module documentations
  • In-code documentation looks appropriate