checkseapp, seapp_contexts: drop sebool= support.

SELinux policy booleans are prohibited in AOSP, so we can drop the
support for the sebool= input selector.

Change-Id: I5ae31247b2f68d90f6ae4c8830458f22c4ffc854
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>