Remove zygote security class declaration.

All uses were removed by I1c925d7facf19b3953b5deb85d992415344c4c9f;
this is just a dead definition.

Change-Id: Id6b08b624c9eea824f5a55d99b7a4ebf9c9f207e
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

security_classes

@@ -132,7 +132,6 @@ class db_sequence		# userspace
 class db_language		# userspace
 
 class binder
-class zygote
 
 # Property service
 class property_service          # userspace