remove //c10:headers (#98420)
The c10 library is light enough that there's not really much benefit
to being very unbazel-y and providing an incomplete library that lacks
the source files.
Differential Revision: [D44713077](https://our.internmc.facebook.com/intern/diff/D44713077/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98420
Approved by: https://github.com/ezyang
