pybind/rados: remove rados.c from MANIFEST.in

pybind/rados: remove rados.c from MANIFEST.in

this fixes the warning of

```
running egg_info
creating
/srv/autobuild-ceph/gitbuilder.git/build/build/src/pybind/rados/rados.egg-info
writing
/srv/autobuild-ceph/gitbuilder.git/build/build/src/pybind/rados/rados.egg-info/PKG-INFO
writing top-level names to
/srv/autobuild-ceph/gitbuilder.git/build/build/src/pybind/rados/rados.egg-info/top_level.txt
writing dependency_links to
/srv/autobuild-ceph/gitbuilder.git/build/build/src/pybind/rados/rados.egg-info/dependency_links.txt
writing manifest file
'/srv/autobuild-ceph/gitbuilder.git/build/build/src/pybind/rados/rados.egg-info/SOURCES.txt'
warning: no files found matching 'rados.c'
```

in the out-of-source tree build, rados.c is not located in the same
directory of setup.py, rados.pyx, and rados.pyd. it is in the build_dir,
for example, it could be:

  ceph/build/src/pybind/rados/pyrex/rados.c

where ceph/build is the build directory of cmake.
and sdist will include it when running egg_info, because cython's
build_ext module lists `rados.c` as one of the "source_files".

Signed-off-by: Kefu Chai <kchai@redhat.com>