The UCF file has three pairs of lines related to clocks which are present on the board, but if your code does not refer to them, you will get the errors you see.
Comment out those lines and that bit will be fine. Don't forget to uncomment them when you start using the clocks (and a flashing LED should be your next test
You also need to uncomment the lines for your LEDs, otherwise they'll be assigned to random pins, which will probably not be very visible!
Finally, when you have a bitfile, check the .pin file that is written out during the build to make sure the signals are on the pins you expected, and that you haven't missed any which are then randomly assigned. My build script has a post-PAR check of the pin file which looks for the tag that goes on unconstrained pins and ERRORs out if it finds one.
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