software/rapid-cdn: remove some unused versions

 - pycrypto does not seem used (because it does not work on py3 with buildout3)
 - cns.recipe.symlink and PyRSS2Gen versions are defined in stack/slapos.cfg

software/rapid-cdn/software.cfg

@@ -217,10 +217,7 @@ zc.lockfile = 1.4
 
 python2-secrets = 1.0.5
 validators = 0.12.2
-PyRSS2Gen = 1.1
-cns.recipe.symlink = 0.2.3
 ecdsa = 0.13
-pycrypto = 2.6.1
 
 furl = 2.1.0
 orderedmultidict = 1.0.1