Revert "[vm] Enable experimental VM service by default"

This reverts commit 5a06139ae705dcf8449e6efc607a1ce86ca30f4c.

Reason for revert: Multiple configurations red

Original change's description:
> [vm] Enable experimental VM service by default
>
> Flips the C++ --enable-experimental-vm-service flag to default to
> enabled (accessible via --disable-experimental-vm-service instead), and
> configures the LUCI test matrix to build all targets with experimental
> VM service support.
>
> This change also updates the analyzer integration tests to copy the new
> VM Service snapshot to the mock SDK to ensure the mock SDK is fully
> functional when the VM service is initialized by default.
>
> TEST=VM service test suite
> Change-Id: I5220fceb3fbd3ee44179b5c705fdfccb627fe71d
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/514480
> Commit-Queue: Ben Konyi <bkonyi@google.com>
> Reviewed-by: Nate Biggs <natebiggs@google.com>

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Change-Id: I13dff176ef8562c3927256f40819648d1e2b8d65
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/522940
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
Reviewed-by: Nate Biggs <natebiggs@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>