how to get rid of arm64 in apt

You can remove a foreign architecture by running

sudo dpkg --remove-architecture arm64

After that, you need to update your software lists.

sudo apt update

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If you still get some errors or warnings, you can try deleting all your software lists and completely re-downloading them from the server, to make sure nothing old is left. Note that the complete download will take a bit longer than usual:

sudo rm -rf /var/lib/apt/lists
sudo apt update

Faced same issue while working with jetson-tx2 arm64. This manual refers the solution in advanced setup scenario It turns out you can't remove architecture without removing packages.

This is useful when a foreign architecture has been added, causing "404 Not Found" errors to appear when the repository meta-data is updated. For example, if you wanted to restrict a repository to only the amd64 and i386 architectures, it would look like:

deb [arch=amd64,i386] <url>

Provide the above option for all repositories in "sudo gedit /etc/apt/sources.list"

Now "sudo apt update" and it will fetch only from amd64 and i386.