Drop unneeded Qt metatype declarations

ScanResult has a Q_GADGET macro and thus has automatic coverage.

The explicit declaration macros only add costs to all places using the header, where the metatype declaration will be only needed by code using the types in queued connections.

See also end of section here: https://doc.qt.io/qt-6/qmetatype.html#Q_DECLARE_METATYPE

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