Both Kelly Gray and Marvin are correct.
The negative terminal of this electrolytic cap has been soldered to both ground pins of the power connector. The positive terminal of the cap has been soldered to the +5V supply line feeding the expansion connectors. As was pointed out by Marvin its purpose was to provide additional filtering for the +5V supply likely because the original owner noted the system was becoming "flaky" as additional cards were added.
There is no harm in leaving it, however a sleeve of insulation should be put over the exposed wires on both ends of that cap so they do not accidentally contact the traces on the solder side of the board.