Delete the linkage tests

* **Explanation**: The linkage tests have been causing more issues than they caught issues. And really what we want to test, is that the libraries built for the compiler using CMake don’t have any unexpected dependencies – it doesn’t really matter if the swift-syntax package itself gains a new link dependency.
* **Scope**: This is a test-only change
* **Risk**: Very low, this is a test-only change and has already been removed on the 509 package release of swift-syntax
* **Testing**: n/a
* **Issue**: rdar://122906543
* **Reviewer**: Ben Barham (@bnbarham) on #2189

Author: ahoppen