Stop pretending that TH headers are both C++ and C compatible. (#16059)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16059
Just deleted all __cplusplus ifdef guards; we only ever use
these headers in C++ contexts.
Reviewed By: dzhulgakov
Differential Revision: D13686580
fbshipit-source-id: ce28c4a32f3596bfb17aeeb34904a02899991453
