systemd: fix a dead link under /var/log

Commit 6fe23ff31c0 changed README to a symlink to README.logs, and
install README.logs under systemd doc dir.

But for OE, systemd doc dir is splited into package systemd-doc, when it
is not installed on the target, there will be an dead link:
Eg:
root@intel-x86-64:/var/log# ls -l README
lrwxrwxrwx 1 root root 39 Jun 20 08:57 README -> ../../usr/share/doc/systemd/README.logs
root@intel-x86-64:/var/log# ls -l ../../usr/share/doc/systemd/README.logs
ls: cannot access '../../usr/share/doc/systemd/README.logs': No such file or directory

Meantime, relative path for a symlink also will meet issue like
"No such file or directory"

Since OE have set ForwardToSyslog=yes, this README is not needed.
So remove this symlink from package systemd

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>