proc_macro/bridge: Remove dead code Slice type
proc_macro/bridge: Remove dead code Slice type

See https://github.com/rust-lang/rust/pull/85390#discussion_r662464868

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