[CMake] Remove `CMAKE_.*_OUTPUT_DIRECTORY` (NFCI)

Summary:
Three `CMAKE_.*_OUTPUT_DIRECTORY` variables used to be set in CMake and
referenced in various other parts of the project. However, in r198205
chapuni added a note to "don't set them anymore", and any remaining
references to them were subsequently removed in r198316 and r199592.

Now that the variables are no longer used anywhere, remove them, along
with the comments advising against using them any longer.

Test Plan:
I ran `check-all` and confirmed the tests built and passed.

Reviewers: beanz, chapuni

Reviewed By: beanz

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D38389

llvm-svn: 314550