DOC: Remove orphaned pxd_package.rst
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.

da-woods/cython,c-blake/cython,fabianrost84/cython,fabianrost84/cython,fabianrost84/cython,da-woods/cython,c-blake/cython,c-blake/cython,fabianrost84/cython,scoder/cython,cython/cython,c-blake/cython,cython/cython,scoder/cython,ChristopherHogan/cython,ChristopherHogan/cython,da-woods/cython,cython/cython,fabianrost84/cython,ChristopherHogan/cython,scoder/cython,c-blake/cython,scoder/cython,cython/cython,da-woods/cython