Remove zygote security class declaration.

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

(cherry-pick of commit: a0c9d207)
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

Change-Id: I2e17e522a78120c3658d81035e202aab861a3b22