compiler, reflect, runtime: remove zero field from type descriptor

compiler, reflect, runtime: remove zero field from type descriptor

    Type descriptors picked up a zero field because the gc map
    implementation used it.  However, it's since been dropped by the gc
    library.  It was never used by gccgo.  Drop it now in preparation for
    upgrading to the Go 1.5 library.

    Reviewed-on: https://go-review.googlesource.com/16486

From-SVN: r229546