Remove unnecessary execCommand('undo') from onCut

As we use onCopy to write data to the clipboard and this is
done entirely in the pasteTarget, the view is not modified by
the cut event, so there's nothing to undo. This is left over
from before we rewrote onCopy.

Bug: 58724
Change-Id: I2269e0ec409dc9eaa0826c78468d9cac554e0433
Merge "Remove unnecessary execCommand('undo') from onCut"