Clean up extraneous &mut.
Clean up extraneous &mut.

graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust