Remove "fischerandom" from UCI combo variants

[alwey]

UCI uses boolean option "UCI_Chess960" or combo UCI_Variant value "var chess960"
Boolean option "UCI_Fischerandom" and combo UCI_Variant value "var fischerandom" are not supported.

Resolves #667

This commit avoids an inconsistent behaviour of CC for FRC/Chess960 when an engine (here: Fairy-Stockfish, versions 10.4 and newer) has both boolean option UCI_Chess960 and combo option UCI_Variant and not using value var chess960 (like Sjaak II) but var fischerandom. CC reads var fischerandom and activates FRCBoard but inconsistently issues setoption name UCI_Variant value chess960. The engine then remains in standard chess mode and loses.

148 >stockfish-fairy-10.4(1): setoption name UCI_Variant value chess960

Older versions of Fairy-Stockfish did not have value var fischerandom and thus the boolean option UCI_Chess960 was used without problem.

148 >stockfish-fairy-10.3(0): setoption name UCI_Chess960 value true

This PR removes var fischerandom from the values of UCI_Variant.
Tested as working with Fairy.Stockfish 10.3, Fairy.Stockfish 10.4, Fairy.Stockfish 14 and Sjaak II version 1.3.1.

126 >stockfish-fairy-10.4(0): setoption name UCI_Chess960 value true
...
126 >stockfish-fairy-10.3(1): setoption name UCI_Chess960 value true

140 >stockfish-fairy-14(0): setoption name UCI_Chess960 value true
...
140 >sjaakii(1): setoption name UCI_Variant value chess960

This can be considered as a work-around before more profound solution is in place.
HTH