cython: drop our package

Cython is only used to build pyzmq. In !1189 (merged) I changed the dependency from our cython to upstream recently added python-cython. So it is no longer necessary to maintain this ourself.

Ref:

• https://github.com/openwrt/packages/tree/openwrt-22.03/lang/python/python-cython
• https://github.com/openwrt/packages/tree/openwrt-23.05/lang/python/python-cython

Closes #959 (closed)