This undoes the seriously broken change from Sponge to support Mixins, that breaks
a wide variety of coremods. Given the widespread incompatibility it introduces, it
won't be re-added at 1.7.10.
This reverts commit 8ccfa24764a3f4854f5334c0da1224286175e13b, reversing
changes made to 7c10b93a2ded2799d41b73b67a2766c31e992d8a.