How can I delete "file1.txt" from my repository?

Use git rm.

If you want to remove the file from the Git repository and the filesystem, use:

git rm file1.txt
git commit -m "remove file1.txt"

But if you want to remove the file only from the Git repository and not remove it from the filesystem, use:

git rm --cached file1.txt
git commit -m "remove file1.txt"

And to push changes to remote repo

git push origin branch_name