Inline and remove HumanReadableErrorType::new_emitter.

And likewise with `ColorConfig::suggests_using_colors`. They both have a
single call site. And note that `BufWriter::supports_color()` always
returns false, which enables a small bit of constant folding along the
way.

compiler/rustc_errors/src/json/tests.rs

@@ -1,6 +1,5 @@
 use super::*;
 
-use crate::emitter::ColorConfig;
 use crate::DiagCtxt;
 use rustc_span::BytePos;