Rollup merge of #52505 - alexcrichton:remove-thinlto-hack, r=nikomatsakis

rustc: Remove a workaround in ThinLTO fixed upstream

This commit removes a hack in our ThinLTO passes which removes available
externally functions manually. The [upstream bug][1] has long since been fixed,
so we should be able to rely on LLVM natively for this now!

[1]: https://bugs.llvm.org/show_bug.cgi?id=35736