fix: safely remove messages when changing templates

[marstamm]

Proposed Changes

This PR fixes the following flows:

• Apply a non-message template to a message element. Currently, this would keep the event definition reference in the element.
• Applying a non-message template to a templated message element. Currently, this would result in an error thrown. Root cause can be found here: https://github.com/camunda/camunda-modeler/issues/4357#issuecomment-2205682401

related to https://github.com/camunda/camunda-modeler/issues/4357

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