[Test] Remove c-index-test from the list of substitutions

All the tests are under the clang tests and none should be under llvm moving
forward.

The topic was discussed in this thread:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140428/214905.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208234 91177308-0d34-0410-b5e6-96231b3b80d8