You can actually get back to the default 1:1 f-curve mapping by simply deleting all keyframes from the f-curve (ie, select the curve, press L to select all (linked) points on that curve, press X to delete). While this means that there isn't an f-curve to edit, Blender will assume a 1:1 relationship between driver value and the resultant property to give the same effect as the initial driver f-curve. If there are no Modifiers present then Blender will render the 'curve' as a dashed line rather than the normal 'solid' line.
In order to be able to edit the f-curve again you will need to re-create an actual f-curve. This can be achieved by simply adding a Keyframe to the object property being driven - ie, in the above example by hovering the mouse over the Y location property (shown in purple in the 3D Editor window) and pressing I.
Once you have created a keyframe you will then be able to adjust the f-curve as normal. To get back to the default state, set the Key 'Frame' and 'Value' to [0.0, 0.0], Left Handle to [-1/3, -1/3], Right Handle to [1/3, 1/3], Handle Types to 'Vector'. Then press I in the F-Curve Editor to add a new keyframe to the channel and set this similar to the above, except [1.0, 1.0] for the Frame/Value, [2/3, 2/3] for Left Handle, [4/3, 4/3] for Right Handle.