gnu: Remove python-pykka.

This trivial package is severely outdated and has failed to build for a long time.

* gnu/packages/python-xyz.scm (python-pykka): Remove variable.

gnu/packages/python-xyz.scm

@@ -3363,32 +3363,6 @@ memory usage and transliteration quality.")
 (define-public python2-pyjwt
   (package-with-python2 python-pyjwt))
 
-(define-public python-pykka
-  (package
-    (name "python-pykka")
-    (version "1.2.1")
-    (source
-     (origin
-       (method url-fetch)
-       (uri (pypi-uri "Pykka" version))
-       (sha256
-        (base32
-         "049w3r0mdnnw7xv19jiq7rvls9k7xs73x05b4qs5d6z4vvmgyiz8"))))
-    (build-system python-build-system)
-    (native-inputs
-     `(("python-mock" ,python-mock)
-       ("python-nose" ,python-nose)
-       ("python-gevent" ,python-gevent)
-       ("python-eventlet" ,python-eventlet)))
-    (home-page "https://www.pykka.org/")
-    (synopsis "Pykka is a Python implementation of the actor model")
-    (description
-     "Pykka is a Python implementation of the actor model.
-The actor model introduces some simple rules to control the sharing
-of state and cooperation between execution units, which makes it
-easier to build concurrent applications.")
-    (license license:asl2.0)))
-
 (define-public python-pymsgbox
   (package
     (name "python-pymsgbox")