init: remove support for disabled SELinux

Remove support for androidboot.selinux=disabled. Running with SELinux
disabled is not a supported configuration anymore. SELinux must be
in enforcing in shipping devices, but we also support permissive for
userdebug/eng builds.

Don't try security_setenforce() if we're already in enforcing mode.
A kernel compiled without CONFIG_SECURITY_SELINUX_DEVELOP does
not have a permissive mode, so the kernel will already be enforcing
once the policy is loaded.

Bug: 19702273
Change-Id: I07525a017ddb682020ec0d42e56a2702c053bdeb