misc: remove deprecated stalebot file (#4199)

Refs: https://github.com/probot/stale/pull/430

.github/stale.yml

@@ -1,23 +0,0 @@
-# Number of days of inactivity before an issue becomes stale
-daysUntilStale: 28
-# Number of days of inactivity before a stale issue is closed
-# Set to false to disable. If disabled, issues still need to be closed
-# manually, but will remain marked as stale.
-daysUntilClose: false
-# Issues with these labels will never be considered stale
-exemptLabels:
-  - v2
-  - enhancement
-  - good first issue
-  - feature-request
-  - doc
-  - bug
-  - not-stale
-# Label to use when marking an issue as stale
-staleLabel: stale
-# Comment to post when marking an issue as stale. Set to `false` to disable
-markComment: >
-  This issue has been automatically marked as stale because it has not had
-  recent activity. Thank you for your contributions.
-# Comment to post when closing a stale issue. Set to `false` to disable
-closeComment: false