Removing the empty signature-certificate-file option.

It is not required to have this option set into the stack.

According to the policy adopted this must be overridden by the
software developer.

stack/shacache-client.cfg

@@ -4,4 +4,3 @@ networkcache-section = networkcache
 [networkcache]
 download-cache-url = http://www.shacache.org/shacache
 download-dir-url = http://www.shacache.org/shadir
-signature-certificate-file =