Drop extra _system_file types.

They serve no purpose; these directories/files are normally accessible
in the same way as the rest of /system.  Also one of them has the wrong
attributes (data_file_type), thereby making it writable by some domains,
and under current policy, shell and apps cannot do ls -l /etc/ppp /etc/dhcpcd.

Change-Id: I0c1baa434fe78373684f4eaab40a41fddf2bdd79
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>