Deleting songs on computer from an iTunes playlist
In iTunes, select the song in your playlist and press ⌘ cmd + ⌥ alt/option + ← backspace.
It will prompt you to delete the file from your iTunes Media folder as well as the playlist itself.
To do this completely from the keyboard, enable Full Keyboard Access to All Controls (from System Preferences → Keyboard → Shortcuts) to be able to tab to the Delete File button, then press Space to 'click' it.
Alternatively, you could create an AppleScript or Automator action that runs on a BetterTouchTool shortcut/gesture to do this in one step.