[ExtractionResult] Remove automatic mimetype determination
ExtractionResult is used as a base class extended by e.g. SimpleExtractionResult in KFM and Baloo::Result and should be a minimal storage class.
The automatic mime type determination fallback is not mentioned in the API description in any way; it is not tested; it is not used by either KFM itself or e.g. Baloo; and it is not failsafe, as the url may be unreachable for Qt.
Remove it as it is essentially dead code.