[DebugInfo] Remove old decls when converting DI (#194964)
We were trying to remove declarations of old debug intrinsics whenever
printing modules or writing them to file. This is no longer necessary as
we use the new-style debug values exclusively now, other than when a
target pass specifically converts back to the old style. If a target
pass does that, removing the intrinsics is not right as the intrinsics'
users will still linger.
This change should be NFC except for the experimental DirectX target
where we do exactly that.
Fixes #194884
