[util] Avoid compiler warning on gcc 4.6

[util] Avoid compiler warning on gcc 4.6

Commit 196751c ("[build] Enable warnings when building utilities")
revealed a previously hidden compiler warning in util/nrv2b.c
regarding an out-of-bounds array subscript in the code

    #if defined(SWD_BEST_OFF)
        if (s->best_pos[2] == 0)
            s->best_pos[2] = key + 1;
    #endif

where best_pos[] is defined by

    #define SWD_BEST_OFF 1

    #if defined(SWD_BEST_OFF)
        unsigned int best_off[ SWD_BEST_OFF ];
        unsigned int best_pos[ SWD_BEST_OFF ];
    #endif

With SWD_BEST_OFF set to 1, it can be proven that all code paths
referring to s->best_off[] and s->best_pos[] will never be executed,
with the exception of the two lines above.  Since these two lines
alone can have no effect on execution, we can safely undefine
SWD_BEST_OFF.

Verified by comparing md5sums of bin/undionly.kpxe before and after
the change.

Signed-off-by: Michael Brown <mcb30@ipxe.org>