[KGA-89] dev: remove useless computation in uint256_fast_exp" (#1603)

removes a useless line that compute `uint256_fast_exp` one too many
times.

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cairo_zero/utils/uint256.cairo

@@ -366,7 +366,6 @@ func uint256_fast_exp{range_check_ptr}(value: Uint256, exponent: Uint256) -> Uin
         return res;
     }
 
-    let pow = uint256_fast_exp(value, half_exponent);
     let (res, _) = uint256_mul(pow, pow);
     return res;
 }