Disable nix-shell based checks

We are essentially testing the same thing in the Flake check, the
difference is in the nix-shell vs `nix develop` invocation.

I will admit (even though this is my making) that it's a bit silly to
essentially test the same thing twice.

.github/workflows/test-develop-classic.yml

@@ -1,21 +0,0 @@
-name: "Development workflow - nix-shell"
-on:
-  pull_request:
-  push:
-    branches:
-      - main
-jobs:
-  tests:
-    strategy:
-      matrix:
-        os: [ ubuntu-latest, macos-latest ]
-    runs-on: ${{ matrix.os }}
-    steps:
-    - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v17
-      with:
-        nix_path: nixpkgs=channel:nixos-unstable
-    - name: Build
-      run: nix-shell --run 'meson build && cd build && ninja'
-    - name: Run tests
-      run: nix-shell --run 'pytest ./tests'

.github/workflows/test-develop-flakes.yml

@@ -1,4 +1,4 @@
-name: "Development workflow - flakes"
+name: "Development workflow"
 on:
   pull_request:
   push: