[libtooling] Remove system headers from test files

Summary:
[libtooling] Remove system headers from test files

System headers from a different platform are likely to make the libtooling tests fail.
In the case of c++, they also make the tests very slow.

I did not find any alternative to `hello_world.cpp` (that would not be in C), so I just removed this test.

Test Plan: make -C libtooling test && make clang-ocaml test

Reviewers: irp

Reviewed By: irp

Differential Revision: https://phabricator.fb.com/D1469139