Replace shadow fields with getters and setters in MixinLivingEntity.

[TheFloydman]

In an attempt to reduce the amount of mixins, this pull request removes the shadow fields from MixingLivingEntity and instead uses the public getters and setters available in LivingEntity.

[qouteall]

Well seems that github does not show the contribution guide https://github.com/qouteall/ImmersivePortalsModForForge/blob/1.16/CONTRIBUTING.md

[TheFloydman]

Cool, I'll make one there when I get an environment set up.