I don´t think doing this via CSS is a good option. I have tried this before and it is possible to remove the VISUAL icon but then you still have tooltip, action etc. and related to this is also the place of the icon e.g. if the export icon is between 2 other icons you have a gap etc..
There is a advanced script option you can use but how and if it will work depends on which other buttons you need in the query/form. I have posted a couple of solutions in the post below (one that removes everything from the query/form header and another that removes all and adds save/restore etc.) and if the existing alternatives don´t work you can possible find/test your own variations.