chore: remove the stale bot configuration file

We are not using https://github.com/apps/stale in this repository, so remove this useless file.
Note: it was used in the original mxGraph GitHub repository.

.github/stale.yml

@@ -1,18 +0,0 @@
-# Number of days of inactivity before an issue becomes stale
-daysUntilStale: 150
-# Number of days of inactivity before a stale issue is closed
-daysUntilClose: 1
-# Issues with these labels will never be considered stale
-exemptLabels:
-  - pinned
-  - security
-  - notstale
-# Label to use when marking an issue as stale
-staleLabel: wontfix
-# 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. It will be closed if no further activity occurs. Thank you
-  for your contributions.
-# Comment to post when closing a stale issue. Set to `false` to disable
-closeComment: false