Remove -Zemit-thin-lto flag

[bjorn3]

As far as I can tell it was introduced in #98162 to allow fat LTO with -Clinker-plugin-lto. Since #136840 we already automatically disable ThinLTO summary generation when -Clinker-plugin-lto -Clto=fat is used, so I think we can remove it. Seahorn is the only user I found on github. I asked in seahorn/verify-rust#6 to confirm my understanding that -Zemit-thin-lto is no longer necessary.