[CMake][NFC] Remove unused logic to choose swift component

[bulbazord]

add_swift_target_library should always have TARGET_LIBRARY set. A few
lines down, there is a precondition that makes sure that it is set. This
check and subsequent assignment of SWIFTLIB_INSTALL_IN_TARGET should
never get triggered.

cc @compnerd @gottesmm @Rostepher

[bulbazord]

@swift-ci please smoke test

[gottesmm]

Can you hoist the precondition check to the top and then remove all references to it?

[bulbazord]

Can you hoist the precondition check to the top and then remove all references to it?

I can hoist the precondition but what do you mean "remove all references to it"?

[gottesmm]

@xiaobai sorry I meant can we just eliminate it? And debride. Sorry I was typing quickly.

[Rostepher]

@gottesmm do you mean removing the TARGET_LIBRARY option altogether? That would be a much larger change, but possibly sensible since this function is now focused completely on the target library use-case.

[compnerd]

@Rostepher - yeah, this was something that @gottesmm and I had chatted about. The desire is to remove the flag since it should never be used for non-target libraries. Yes, it is a larger change, but is part of the simplifications that will pay large dividends as we start pruning all the custom handling of build rules.

@gottesmm I think that removing the flag in a follow up is a reasonable approach. This just removes the unnecessary variable set.

[gottesmm]

Yes. Everything that is passed there should have TARGET_LIBRARY set. That means we should debride.

[gottesmm]

@compnerd I am fine with that. As long as the follow up commit happens.