Remove backport from CDT releng (#1089)

It was useful when we were maintaining two branches, but
now it causes numerous false negatives in PRs

Reverts https://github.com/eclipse-cdt/cdt/pull/849

.backportrc.json

@@ -1,9 +0,0 @@
-{
-  "repoOwner": "eclipse-cdt",
-  "repoName": "cdt",
-  "targetBranchChoices": ["cdt_11_6"],
-  "branchLabelMapping": {
-    "^backport-to-(.+)$": "$1"
-  },
-  "multipleCommits": true
-}

.github/workflows/backport.yml

@@ -1,25 +0,0 @@
-name: Automatic backport action
-
-on:
-  pull_request_target:
-    types: ["labeled", "closed"]
-
-jobs:
-  backport:
-    name: Backport PR
-    runs-on: ubuntu-latest
-    steps:
-      - name: Backport Action
-        uses: sorenlouv/backport-github-action@v9.5.1
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          auto_backport_label_prefix: backport-to-
-          add_original_reviewers: true
-
-      - name: Info log
-        if: ${{ success() }}
-        run: cat ~/.backport/backport.info.log
-
-      - name: Debug log
-        if: ${{ failure() }}
-        run: cat ~/.backport/backport.debug.log