Can I shorten a length of C9 Christmas lights?
Each LED has a specific forward voltage drop (it depends on color and power etc) if you remove some from the middle of a series, the overall voltage drop will be less. This means that you need to reduce the voltage applied to the diminished series.

You don't show what is powering these LEDs. It is possible to use a constant-current circuit that would avoid this sort of problem.

If you know (or can measure) the LED current for each LED (end therefore for a series) and the voltage drop of each and the applied voltage, you could calculate the resistance and wattage of a resistor to limit the current to  a safe value.

$$ R = { V_{supply} - n  V_{LED} \over I_{LED} } $$

$$ P = {I_{LED}}^2 R $$