Click on the Trash can on the Dock, hold down the ‘option’ key and click the ‘Empty’ button over there on the left side of the window. If this doesn’t do it, the file may be in the .Trash folder of your Time Machine (TM) or some other disk.

To find out if that’s so, follow this procedure:
 
1. Open Terminal.app, copy and paste the following command into the Terminal window
 
defaults write com.apple.finder AppleShowAllFiles TRUE; killall Finder
 
Then press ‘Return’.
 
2. Open a Finder window. Navigate to the TM disk starting from its icon in  the left hand column. You should see some greyed out folders called .Trash and .Trashes. Click on these and have a look for the boot.efi file that we’re hunting down.
  
To remove the boot.efi file from the hidden trash, try the following:
 
3. Go back to Terminal and copy and paste the following:
 
sudo rm -rf
 
Do NOT press ‘Return’. Instead, press the Spacebar once, then use your cursor to drag the boot.efi file from the hidden folder in Step 2 and drop it in the Terminal window. Now press ‘Return’. You will be asked for an administrator password and given a warning which you can ignore. Type in your password, but notice that your typing will be invisible, so type carefully.

Press ‘Return’.
 
4. If you typed your password incorrectly, repeat step 3. If you typed it correctly, hopefully, your Trash is empty. 🙂
 
5. The last thing is to hide all the hidden files again, so copy and paste this:
 
defaults write com.apple.finder AppleShowAllFiles FALSE; killall Finder
 
Then press ‘Return’.
 
You can close Terminal now.
 
 
Good luck!