Delete disabled cross module incremental tests

The legacy driver doesn't fully support cross module incremental builds.
Some of these tests were disabled, but in general they have been
mirrored to the swift-driver test suite. https://github.com/apple/swift-driver/tree/f6fde8c30a0a01bfbf83fe4e0f2d7994a0c0ac32/Tests/IncrementalImportTests

Author: keith