ci: Remove legacy-os test.

As of yesterday, the GitHub Actions ubuntu-16.04 environment has been
removed.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.github/workflows/legacy-os.yml

@@ -1,24 +0,0 @@
-name: Legacy OS
-
-on: [push, pull_request]
-
-jobs:
-  xenial:
-    name: Ubuntu 16.04 Xenial (Python 3.5, legacy)
-    runs-on: ubuntu-16.04
-    steps:
-      - uses: actions/checkout@v2
-      - name: Check tools/provision error message on xenial
-        run: |
-          { { ! tools/provision 2>&1 >&3; } | tee provision.err; } 3>&1 >&2
-          grep -Fqx 'Error: ubuntu 16.04 is no longer a supported platform for Zulip.' provision.err
-      - name: Check scripts/lib/upgrade-zulip-stage-2 error message on xenial
-        run: |
-          { { ! sudo scripts/lib/upgrade-zulip-stage-2 2>&1 >&3; } | tee upgrade.err; } 3>&1 >&2
-          grep -Fq 'upgrade-zulip-stage-2: Unsupported platform: ubuntu 16.04' upgrade.err
-
-      - name: Report status
-        if: failure()
-        env:
-          ZULIP_BOT_KEY: ${{ secrets.ZULIP_BOT_KEY }}
-        run: tools/ci/send-failure-message