Revert "allow the 2.9.x stable sdk in the meta null_safety prerelease"

This reverts commit 0a2f674e282d16bea7d68dbd08a9b2ed943e21d3.

Reason for revert: This accidentally allows 2.10 dev releases as well

Original change's description:
> allow the 2.9.x stable sdk in the meta null_safety prerelease
> 
> Change-Id: I383e630cf4edba19e4b8ee38adfb0c276eff1d33
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154750
> Auto-Submit: Jake Macdonald <jakemac@google.com>
> Commit-Queue: Jake Macdonald <jakemac@google.com>
> Commit-Queue: Nate Bosch <nbosch@google.com>
> Reviewed-by: Nate Bosch <nbosch@google.com>

TBR=jakemac@google.com,nbosch@google.com

Change-Id: I3461eeda4155ca40e4a80cc492b61d10d6c83646
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154820
Reviewed-by: Jake Macdonald <jakemac@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>