[workflow] removed unused assign reviewer workflow (#2318)

.github/reviewer_list.yml

-addReviewers: true
-
-addAssignees: author
-
-numberOfReviewers: 1
-
-reviewers:
-  - frankleeeee
-  - kurisusnowdeng

.github/workflows/assign_reviewer.yml

-name: Assign Reviewers for Team
-
-on: 
-  pull_request:
-    types: [opened]
-
-jobs:
-  assign_reviewer:
-    name: Assign Reviewer for PR
-    runs-on: ubuntu-latest
-    if: |
-      github.event.pull_request.draft == false && github.base_ref == 'main'
-      && github.event.pull_request.base.repo.full_name == 'hpcaitech/ColossalAI' 
-      && toJson(github.event.pull_request.requested_reviewers) == '[]'
-    steps:
-      - uses: kentaro-m/auto-assign-action@v1.2.1
-        with:
-          configuration-path: '.github/reviewer_list.yml'