Elligator script: removed erroneous .abs()

Elligator script: removed erroneous .abs()

Changing the sign of the v coordinate had an effect on the final value
of the final hash, but wasn't detected because my initial tests only
compare to the u coordinate, which appears to be correct.

This doesn't affect the success or failure of the Elligator mapping,
which only look at the u coordinate.  Yet another example of incorrect
crypto that looks like it works...