[infra] Remove win SDK builder test matrix entry.
[infra] Remove win SDK builder test matrix entry.

Both the production and try builders for the Windows SDK have been
migrated to the new recipe that doesn't use the test matrix.

Bug: b/247507699
Change-Id: I7a603e716492ffa272a43eea959b721c8c4a8f1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259842
Commit-Queue: Jonas Termansen <092ec58085ae5e368bc39d18752740ff81a2d09c@google.com>
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk