Fix SyntaxWarning in GtkSettingsEditor._get_keyfile

[leigh123linux]

Move return statement out of finally block to eliminate SyntaxWarning.
The finally block served no cleanup purpose and has been removed.

[leigh123linux]

$ cinnamon-settings
KeybindingWidgets session type: x11
/usr/share/cinnamon/cinnamon-settings/bin/CinnamonGtkSettings.py:44: SyntaxWarning: 'return' in a 'finally' block
  return keyfile
Using pam module (python3-pampy)

[mtwebster]

I think you need to unindent this line - otherwise this method will only ever return None (on error) or fall through and also return nothing.