I found the question interesting enough to examine it further, especially since it provoked another round of anti-2D sentiment.
The command piecewise(x < 1, a, x < 2, b, x < 3, c, d); creates a piecewise function that I actually entered from the piecewise template in the Expression palette (using 2D, or typeset, math). Now, what does it mean to delete a row? Suppose it is the second row that is to be deleted. What function value takes the place of "b" on the interval [1,2)? I think that's the essential question. Is the function to be undefined on that subinterval? or is the value to be, say, "c" on that subinterval? So, just deleting the row isn't enough. One has to determine what is to happen when the condition in that row is removed.
For example, to have the function assume the value "c" on that subinterval, I used Context Menu/ Evaluate at a Point, and set b=c, then invoked Simplify/Simplify from the Context Menu. The result was the piecewise function that could be described by the command-form piecewise(x < 1, a, x < 3, c, 3 <= x, d);
But the OP might have meant the following work-flow. The Piecewise template is entered from the Expression palette and a row is added. Immediately, it is recognized that this row is not wanted. Control-z will undo the insertion of the row.
What other scenario could have provoked the original question?