How to remove RSS Feed-icon?
if you don't want to disable the feed, but just remove the icon.

According to FiNeX comment in Disable RSS icon in Drupal 8 via Views > Frontpage


  A better way to remove the icon leaving the RSS feed active is to
  simply disable the feed attachment option.
  
  1) Structure > Views > Frontpage Edit
  
  2) Click on "Feed" tab display
  
  3) Click on "Page" next to "Attach" 
  
  4) Uncheck "Page" 
  
  5) Save




Now if you want to disable RSS, then 

According to couturier in Disable RSS icon in Drupal 8 via Views > Frontpage


  Go to Structure > Views > Frontpage > Edit
  
  Then click on "Feed" at the top. In the second drop-down box, select
  "Disable Feed." Then "Save" the change.