debian: remove ${shlibs:Depends} from python-* packages

debian: remove ${shlibs:Depends} from python-* packages

* debian/control:
  as we have listed the linked libraries in Depends section, for example,
  python-rados depends on librados. and we don't need `dpkg-shlibdeps` to
  help figure out shared library substvar dependencies for us. by removing
  them, we can silence the warnings of
```
warning: dpkg-shlibdeps: package could avoid a useless dependency if
debian/python-rados/usr/lib/python2.7/dist-packages/rados.x86_64-linux-gnu.so
was not linked against libpthread.so.0 (it uses none of the library's
symbols)
```
  -lpthread is introduced by `python-config --ldflags` but it turns out we
  are not using any symbols from pthread in the extension directly. and
  pthread is included in glibc. so this does not added any extra
  dependency to python-* pacakges. but it's desirable to have less
  warnings.
* debian/rules: exclude python-* packages from dh_shlibdeps, as we will
  not use it to prepare the shlib deps substvars for these packages any
  more.

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