Remove outdated documentation

[bouweandela]

Description

This removes some outdated documentation that I overlooked in #2202. This should also fix the documentation build.

Link to documentation: https://esmvaltool--2210.org.readthedocs.build/projects/ESMValCore/en/2210/recipe/preprocessor.html

Closes #2211

---

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🧪 and 🛠 Documentation is available
• 🛠 Changes are backward compatible
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov]

Codecov Report

Merging #2210 (68bb814) into main (6900168) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2210   +/-   ##
=======================================
  Coverage   93.21%   93.21%           
=======================================
  Files         238      238           
  Lines       12851    12851           
=======================================
  Hits        11979    11979           
  Misses        872      872