DOC: Remove orphaned pxd_package.rst

This rst file is not referenced anywhere in the docs and can only be
accessed by going directly to
http://docs.cython.org/src/userguide/pxd_package.html.  It also seems to
be a remnant from the Pyrex -> Cython merge.

docs/src/userguide/pxd_package.rst

-I think this is a result of a recent change to Pyrex that
-has been merged into Cython.
-
-If a directory contains an :file:`__init__.py` or :file:`__init__.pyx` file,
-it's now assumed to be a package directory. So, for example,
-if you have a directory structure::
-
-   foo/
-     __init__.py
-     shrubbing.pxd
-     shrubbing.pyx
-
-then the shrubbing module is assumed to belong to a package
-called 'foo', and its fully qualified module name is
-'foo.shrubbing'.
-
-So when Pyrex wants to find out whether there is a `.pxd` file for shrubbing,
-it looks for one corresponding to a module called `foo.shrubbing`. It
-does this by searching the include path for a top-level package directory
-called 'foo' containing a file called 'shrubbing.pxd'.
-
-However, if foo is the current directory you're running
-the compiler from, and you haven't added foo to the
-include path using a -I option, then it won't be on
-the include path, and the `.pxd` won't be found.
-
-What to do about this depends on whether you really
-intend the module to reside in a package.
-
-If you intend shrubbing to be a top-level module, you
-will have to move it somewhere else where there is
-no :file:`__init__.*` file.
-
-If you do intend it to reside in a package, then there
-are two alternatives:
-
-1. cd to the directory containing foo and compile
-   from there::
-
-      cd ..; cython foo/shrubbing.pyx
-
-2. arrange for the directory containing foo to be
-   passed as a -I option, e.g.::
-
-      cython -I .. shrubbing.pyx
-
-Arguably this behaviour is not very desirable, and I'll
-see if I can do something about it.
-