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>