Shortcut to remove all keyframes from a clip
You can delete the keyframes quickly by selecting them in the Effect Controls panel. 



On the right side of the panel, you can see the mini timeline containing all the keyframes at their corresponding positions. First, double-click the bar at the bottom to make sure the entire clip area is visible. Then, you can drag a marquee selection with your mouse, or click anywhere in the keyframe area and press CTRL + A to select all keyframes. Then simply hit backspace on your keyboard to delete the selected keyframes. If you delete all the keyframes for an effect parameter, this will also reset this parameter to the last value it had before you activated the animation.

Note that this does not deactivate the animation for that parameter (i.e. the stopwatch icon stays highlighted). So if you change the parameter value after that, it will create a new keyframe at the current playhead position. Press the stopwatch icon to deactivate the animation for the corresponding parameter. If the parameter has no keyframes, the warning prompt will not pop up.