Remove old iPadMini4 jobs.

Remove old iPadMini4 jobs.

These were replaced with RPi-based jobs in
https://skia-review.googlesource.com/c/11521/. The old jobs no longer
have any bots to run them, which is causing a lot of warnings in the
task scheduler logs.

No-Try: true
Change-Id: Iae7b8371cd80fd7ee3fbe368d5dccb0a94913f14
Reviewed-on: https://skia-review.googlesource.com/16876
Reviewed-by: Eric Boren <borenet@google.com>
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>