Remove dead code from setup.py, remove need for build target. (#16162)

Remove dead code from setup.py, remove need for build target. (#16162)

Summary:
Now it is only necessary to use 'develop' or 'install' to build. Incremental cmake is on by default. `develop --cmake` forces it to rerun.

The NinjaBuilder stuff is dead. It was used to make building _C.so
faster but now _C.so is just an empty stub file.

Removed a bunch of custom build commands from setup.py that are
no longer meaningful now that cmake handles most of the build.

Removed unused targets in build_pytorch_lib.sh/bat
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16162

Differential Revision: D13744155

Pulled By: zdevito

fbshipit-source-id: d836484782c65b7f8e8c7a82620886f7a7777892