Revert "Add check-rebased Github action (#2002)"

This reverts commit 2104330d4c.

This check doesn't work well for community pull requests, since third-party users
aren't motivated to rebase pull requests to branch head after they are created.

This check is useful for private repositories though.

.github/workflows/check-rebased.sh

@@ -1,26 +0,0 @@
-#!/usr/bin/env bash
-set -x -e -u
-
-git fetch origin "$GITHUB_BASE_REF"
-
-BASE_ID=$(git rev-parse --verify "origin/$GITHUB_BASE_REF")
-CURRENT_ID=$(git rev-parse --verify "HEAD")
-
-# Oldest commit that's not in the base branch
-FIRST_BRANCH_ID=$(git rev-list "origin/$GITHUB_BASE_REF..HEAD" | tail -n 1)
-
-# Common ancestor (usually just parent of $FIRST_BRANCH_ID)
-# Command will return error 1 if not found anything. So we || true to proceed.
-FORK_POINT_ID=$(git merge-base "$FIRST_BRANCH_ID" "origin/$GITHUB_BASE_REF") || true
-
-if [[ -z "$FORK_POINT_ID" ]]
-then
-  echo "Current branch is not forked from its base branch origin/$GITHUB_BASE_REF"
-  exit 1
-fi
-
-if [[ "$BASE_ID" != "$FORK_POINT_ID" ]]
-then
-  echo "Current branch (at $CURRENT_ID) forked at $FORK_POINT_ID is not $BASE_ID (which is the latest \"$GITHUB_BASE_REF\")"
-  exit 1
-fi

.github/workflows/check-rebased.yml

@@ -1,17 +0,0 @@
-name: check-rebased
-on:
-  pull_request:
-    types: [ synchronize, opened, reopened, edited ] # +edited (for triggering on PR base change)
-  push:
-    branches: [ main ]
-jobs:
-  hook:
-    name: Check whether current branch is based on its base branch
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check out repo @ current branch
-        uses: actions/checkout@v2
-        with:
-          fetch-depth: 1000 # Hopefully current branch is less than 1000 commits from main
-      - name: Run check-rebased.sh
-        run: .github/workflows/check-rebased.sh