Revert "Forward compatibility for renaming the be channel to main."

This reverts commit 90300b9ca1cbf110725a21ae1a752922d2024ac7.

Reason for revert: A bunch of purple bots mentioning:
`NameError: name 'IncreasedNumberOfFileDescriptors' is not defined`.

Original change's description:
> Forward compatibility for renaming the be channel to main.
>
> Delete lots of dead code in bot_utils and utils. The logic has been
> migrated into the recipes for a long time already or has otherwise
> become obsolete.
>
> Bug: b/270022416
> Change-Id: I7977c7a0d129e6112ad143f10f554aec7d9f653f
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/326780
> Commit-Queue: Jonas Termansen <sortie@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>

Bug: b/270022416
Change-Id: I3b008eeb2c729fce4e9abc5879c91d2416dfa692
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/327960
Reviewed-by: Martin Kustermann <kustermann@google.com>
Auto-Submit: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>