if you dont push your changes to git yet
git reset --soft HEAD~1
It will reset all the changes and revert to one commit back
If this is the last commit you made and you want to delete the file from local and remote repository try this :
git rm <file>
git commit --amend
or even better :
reset first
git reset --soft HEAD~1
reset the unwanted file
git reset HEAD path/to/unwanted_file
commit again
git commit -c ORIG_HEAD