We can remove the systemd options since the directory is managed by
tmpdir, and the mount has the needed "z" flag for relabelling.
The RuntimeDirectory instructs systemd to create AND remove the
directory - this might lead to an issue where init_t isn't allowed to
unlink a container_file_t file/directory
The ExecStartPre is running in init_t context, and might be denied to
chcon file/directory to container_file_t.
It should resolves rhbz#1860423 while making things cleaner and clearer.
This is also the only place where we actually use the
`systemd_exec_flags`.
Change-Id: Ie938d9dd7d74db3907eb546765236b025bff1abe
9b894ecb69