Remove legacy existing property from IContainerContext

[andre4i]

See #3429

As IRuntimeFactory.instantiateRuntime has an existing parameter which can be used to check for this state, the existing property on IContainerContext is redundant. This should be done after we're able to safely do a breaking change in IRuntimeFactory (@fluidframework/container-definitions) by making the existing parameter mandatory.

Until the back-compat changes go in, we could deprecate the property by asserting its use, to ensure there is no current client using it.

Acceptance criteria:

• existing in IRuntimeFactory.instantiateRuntime is mandatory
• existing property from IContainerContext/ContainerContext is removed
• RuntimeFactoryHelper is simplified to not check context.existing
• ContainerRuntime is simplified to not check context.existing

[ghost]

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