The only way to safely remove a solar panel is to power it down and disconnect it from the array. After that, you can turn off the solar connection and should. Remember that solar panels are a circuit so that energy can flow away or towards the panel.
One method of removal, if you have used that option, is to carefully slice with a razor or equally thin sharp cutting device between the wall and the cup mount. Once the body of the adhesive is split, additional scraping and application of adhesive removal product such as Goo-Gone will eliminate the residue.
If the panel is grid-tied, you will need to also stop the flow of energy from the grid in case it should flow in reverse and enter the panel. With the solar connections turned off, you can safely disconnect the wiring. Be sure to use a voltage regulator to ensure that the power is off. How do you repair the glass on a solar panel?