Drop unusable SELinux APIs.

setSELinuxEnforce is not allowed by policy.
get/setBooleanValue and getBooleanNames are moot since SELinux
policy booleans are forbidden by CTS.

Change-Id: I512953edd1c38828efce698ae8475603775c32e8
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>