tzdatacheck: don't hard code SELinux domain name

An automatic domain transition is already defined by SELinux
policy. Avoid having redundant information on the exec line.

This commit depends on commit 17fff893
which made the SELinux process label optional.

(cherrypicked from commit 221fca7d)

Change-Id: I89464f2bd218c7d6e8db08aa6bed2b62ec6dad2a