rustc_codegen_ssa: remove some unnecessary Box special-casing.
rustc_codegen_ssa: remove some unnecessary Box special-casing.

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