chore: remove auto-assign bot config file. (#1446)

.github/auto_assign.yml

-# Set to true to add reviewers to pull requests
-addReviewers: true
-
-# Set to true to add assignees to pull requests
-addAssignees: true
-
-# A list of reviewers to be added to pull requests (GitHub user name)
-reviewers:
-  - ruibaby
-  - JohnNiang
-  - guqing
-  - LIlGG
-
-# A list of keywords to be skipped the process that add reviewers if pull requests include it
-skipKeywords:
-  - wip
-
-# A number of reviewers added to the pull request
-# Set 0 to add all the reviewers (default: 0)
-numberOfReviewers: 0
\ No newline at end of file