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If you want to remove the start new topic on mobile, desktop forums list and forum index then simply set the user permissions. how are you as admin supposed to add a new topic if theres no button to add new topics? 
and if you hide the gap completely you would also hide  other forum buttons like Table, Grid, Fluid view forums 
 
anyway if you still want to do that add this in custom.css it will hide the page header completely including anything inside and everywhere on forums
body[data-pagemodule="forums"] .ipsPageHeader{display: none;}