Fix segfault sorting runnable steps

Same problem as d744362e4a.

    at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/predefined_ops.h:166
    __last@entry=..., __comp=...) at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/stl_algo.h:1827
    __comp=...) at /nix/store/ksvsbr7pg4z69bv6fbbc8h7x7rm2104m-gcc-4.9.3/include/c++/4.9.3/bits/stl_algo.h:4717

src/hydra-queue-runner/state.hh

@@ -171,10 +171,6 @@ struct Step
            step. */
         int highestGlobalPriority{0};
 
-        /* The lowest share used of any jobset depending on this
-           step. */
-        double lowestShareUsed;
-
         /* The highest local priority of any build depending on this
            step. */
         int highestLocalPriority{0};