Fix a long-standing bug where Synapse would not unbind third-party identifiers...

Fix a long-standing bug where Synapse would not unbind third-party identifiers for Application Service users when deactivated and would not emit a compliant response. (#16617)

* Don't skip unbinding 3PIDs and returning success status when deactivating AS user

Fixes #16608

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

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Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>