Remove support for specializing ToString outside the standard library

This is the only trait specializable outside of the standard library.
Before stabilizing specialization we will probably want to remove
support for this. It was originally made specializable to allow a more
efficient ToString in libproc_macro back when this way the only way to
get any data out of a TokenStream. We now support getting individual
tokens, so proc macros no longer need to call it as often.

compiler/rustc_span/src/lib.rs

@@ -25,7 +25,6 @@
 #![feature(hash_set_entry)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(read_buf)]
 #![feature(round_char_boundary)]