ci: Remove the nightly-rust workflow (#31091)

This workflows tested Servo with the most recent nightly version of
Servo. Now that Serv compiles against a stable version of Rust, this
isn't so interesting. We are hoping to eliminate all use of unstable
features in the future.

.github/workflows/nightly-rust.yml

@@ -1,38 +0,0 @@
-name: Test with nightly rustc
-
-on:
-  schedule:
-    # Run at 5:15 am, daily.
-    - cron: '15 5 * * *'
-  workflow_dispatch:
-
-env:
-  RUST_BACKTRACE: 1
-  SHELL: /bin/bash
-
-jobs:
-  build-linux-with-rust-nightly:
-    # This job is only useful when run on upstream servo.
-    if: github.repository == 'servo/servo' || github.event_name == 'workflow_dispatch'
-    name: Build (Linux) + rustc nightly
-    runs-on: ubuntu-20.04
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          fetch-depth: 2
-      - name: Update rustc
-        run: echo nightly > rust-toolchain
-      - uses: actions/setup-python@v5
-        with:
-          python-version: '3.10'
-      - name: Bootstrap
-        run: |
-          python3 -m pip install --upgrade pip
-          sudo apt update
-          python3 ./mach bootstrap
-      - name: Release build
-        run: python3 ./mach build --release
-      - name: Smoketest
-        run: xvfb-run python3 ./mach smoketest --release
-      - name: Unit tests
-        run: python3 ./mach test-unit --release