Remove setEnforcingMode from SELinuxPolicyInstallReceiver.

It never worked anyhow, at least in AOSP, as nothing checks that property,
and is pointless now that enforcing mode is set by init.

Change-Id: If05dd49963c5d7081e00039b5e378032bea0939b
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>