Remove ability to read all /proc/pid/attr/current entries.

This was rendered obsolete when SELinuxDomainTest was ported
to SELinuxHostTest and only makes sense if allowing search
to domain:dir and { open read } to domain:file in order to
open the /proc/pid/attr/current files in the first place.
SELinux applies a further :process getattr check when
reading any of the /proc/pid/attr/* files for any process
other than self, which is no longer needed by app domains to
pass CTS.

Change-Id: Iff1e601e1268d4d77f64788d733789a2d2cd18cc
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>