Remove bad test.

We currently don't canonicalize paths in the preprocessed files.
But we do when writing to PCH.
This causes a discrepancy on Windows with the test below.
This test fails even on unix if you change the test to use
`%S//preprocess.h`.

I am led to conclude that the invariant that this test was intending to
test has not been upheld for a while (and may never have been).

llvm-svn: 243602