tools: (Actually) drop libsystemd as a dependency

tools: (Actually) drop libsystemd as a dependency

When reinstating some of systemd.m4 between v1 and v2, I reintroduced a little
too much.  While {c,o}xenstored are indeed no longer linked against
libsystemd, ./configure still looks for it.

Drop this too.

Fixes: ae26101f6bfc ("tools: Drop libsystemd as a dependency")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>