Remove support for reading Nix config from flake.

The code that extracted Nix config from flake.nix was simply too
fragile.

action.yml

@@ -45,16 +45,6 @@ inputs:
     description: |
       Don't use. For bootstrapping purposes only.
 
-  load_nixConfig:
-    default: true
-    description: |
-      Reads out a flakes 'nixConfig' attribute, if there is one, and loads
-      it into the environment config for old-style commands (`nix-build`,
-      `nix-store` etc) to pick it up.
-
-      If this cause undesired behaviour, you can switch it off by setting
-      this to false.
-
 
 runs:
   using: "composite"
@@ -70,16 +60,6 @@ runs:
         NIX_ON_TMPFS: ${{ inputs.nix_on_tmpfs }}
         GITHUB_ACCESS_TOKEN: ${{ inputs.github_access_token }}
 
-    - name: Preload nixConfig from Flake
-      run: ${{ github.action_path }}/read-nix-config-from-flake.sh
-      shell: bash
-      env:
-        LOAD_NIXCONFIG: ${{ inputs.load_nixConfig }}
-        NIX_VERSION: ${{ inputs.nix_version }}
-        GH_TOKEN: ${{ inputs.github_access_token }} # used by gh cli
-        OWNER_AND_REPO: ${{ github.repository }}
-        SHA: ${{ github.sha }}
-
 branding:
   icon: zap
   color: gray-dark