Remove pre-expansion AST stats.
They're very little value, because they only measure the top-level `main.rs` or `lib.rs` file. (Other `.rs` files don't get read and parsed until expansion occurs.)
I saw an example recently where the pre-expansion AST was 3KB in size and the post-expansion AST was 66MB.
I kept the "POST EXPANSION" in the output header, I think that's useful information to avoid possible confusion about when the measurement happens.
r? `@davidtwco`
b7150d4e63