Remove docs preview

This is failing CI on Dependabot PRs.  Opted to just remove it since
most (all?) PRs will be from forks and this workflow will not run
anyway.  If we figure out the secret situation we can add it back.

.github/workflows/docs-preview.yaml

-name: Docs preview
-
-on:
-  pull_request:
-    branches:
-      - main
-    paths:
-      - "docs/**"
-
-permissions:
-  actions: none
-  checks: none
-  contents: read
-  deployments: none
-  issues: none
-  packages: none
-  pull-requests: write
-  repository-projects: none
-  security-events: none
-  statuses: none
-
-jobs:
-  preview:
-    name: Docs preview
-    runs-on: ubuntu-20.04
-    steps:
-      - uses: actions/checkout@v3
-
-      - name: Set outputs
-        id: vars
-        run: echo "::set-output name=sha_short::$(git rev-parse --short HEAD)"
-
-      - name: Comment Credentials
-        uses: marocchino/sticky-pull-request-comment@v2
-        # Only run if PR comes from base repo
-        # Reason: forks cannot access secrets and this will always fail
-        if: github.event.pull_request.head.repo.full_name == github.repository
-        with:
-          GITHUB_TOKEN: ${{ github.token }}
-          header: codercom-preview-docs
-          message: |
-            ✨ code-server docs for PR #${{ github.event.number }} is ready! It will be updated on every commit.
-            * _Host_: https://coder.com/docs/code-server/${{ steps.vars.outputs.sha_short }}
-            * _Last deploy status_: success
-            * _Commit_: ${{ github.event.pull_request.head.sha }}
-            * _Workflow status_: https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}