Remove "major" and "p1" from stalebot

LoopBack 3 is in Maintenance LTS mode, only critical bugs and critical
security fixes will be provided. Major and p1 issues are not qualified
and if nobody submits a pull request for them, then they should be
automatically closed as stale.

Author: bajtos

.github/stale.yml

@@ -7,8 +7,6 @@ exemptLabels:
   - pinned
   - security
   - critical
-  - p1
-  - major
 # 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