Revert ".github/workflows: build NixOS/Nixpkgs manuals (#88488)"

This reverts commit b3d674549f.

.github/workflows/manual-nixos.yml

@@ -1,19 +0,0 @@
-name: "Build NixOS manual"
-
-on:
-  pull_request_target:
-    paths:
-      - 'nixos/**'
-
-jobs:
-  nixos:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v11
-      - uses: cachix/cachix-action@v6
-        with:
-          name: nixpkgs-ci
-          signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-      - name: Building NixOS manual
-        run: nix-build nixos/release.nix -A manual.x86_64-linux

.github/workflows/manual-nixpkgs.yml

@@ -1,19 +0,0 @@
-name: "Build Nixpkgs manual"
-
-on:
-  pull_request_target:
-    paths:
-      - 'doc/**'
-
-jobs:
-  nixpkgs:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v11
-      - uses: cachix/cachix-action@v6
-        with:
-          name: nixpkgs-ci
-          signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-      - name: Building Nixpkgs manual
-        run: nix-build pkgs/top-level/release.nix -A manual