Remove unnecessary nested_preloads from RunnersResolver

What does this MR do and why?

This MR removes a buggy nested_preloads overload which actually isn't needed.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

It is evident in the following log comparison that there is no difference after removing the nested_preloads override:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. On the master branch, run the following query on http://gdk.test:3000/-/graphql-explorer and capture the logs (log/development.log) on the 3rd run (to ensure that the run is "clean"):

   {
     runners(type: PROJECT_TYPE) {
       nodes {
         createdBy {
           webPath
           webUrl
         }
         ownerProject {
           fullPath
           webUrl
         }
       }
     }
   }

2. On this branch, re-run and capture the logs again on the 3rd run:

3. Compare the logs (e.g. with Beyond Compare)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.