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.

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