compiletest: Remove the magic hacks for finding output with `lto=thin`
This hack was intended to handle the case where `-Clto=thin` causes the compiler to emit multiple output files (when producing LLVM-IR or assembly).
The hack only affects 4 tests, of which 3 are just meta-tests for the hack itself. The one remaining test that motivated the hack currently doesn't even need it!
(`tests/codegen/issues/issue-81408-dllimport-thinlto-windows.rs`)
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