This removes a useless call to QImage::loadFromData() that speeds up the thumbnail generation for RAW files. It's only purpose was to look up the size of the embedded preview image which is useless because
* RAWs have an embedded thumbnail which will be preferred to the embedded preview image (provided the thumb is available and big enough), and
* if the embedded thumbnail cannot be used, QImageReader::read() will decode the embedded preview image again, check it's size, scale it, etc..
Also removes a redundant comparison against `MIN_PREV_SIZE` as this is
already assured with `pixelSize`.