Remove SmallStr.

It no longer has any uses. If it's needed in the future, it can be
easily reinstated. Or a crate such as `smallstr` can be used, much like
we use `smallvec`.

Author: nnethercote

compiler/rustc_data_structures/src/lib.rs

@@ -68,7 +68,6 @@ pub mod macros;
 pub mod obligation_forest;
 pub mod sip128;
 pub mod small_c_str;
-pub mod small_str;
 pub mod snapshot_map;
 pub mod svh;
 pub use ena::snapshot_vec;