cgroup: get rid of MemorySoftLimit=

The cgroup attribute memory.soft_limit_in_bytes is unlikely to stay
around in the kernel for good, so let's not expose it for now. We can
readd something like it later when the kernel guys decided on a final
API for this.

src/systemctl/systemctl.c

@@ -3657,7 +3657,7 @@ static int append_assignment(DBusMessageIter *iter, const char *assignment) {
                     !dbus_message_iter_append_basic(&sub, DBUS_TYPE_BOOLEAN, &b))
                         return log_oom();
 
-        } else if (streq(field, "MemoryLimit") || streq(field, "MemorySoftLimit")) {
+        } else if (streq(field, "MemoryLimit")) {
                 off_t bytes;
                 uint64_t u;