core: improve log message when unit deactivates cleanly

Whenever a unit deactivates sucessfully we so far generated a message
"unit xyz: succeeded". This is a bit confusing, since various unit types
cannot really "succeed", e.g. a device unit can't really "suceed", not
can a swap unit. In particular in the latter case people would probably
assume that a swap unit that "suceeded" would actually mean a swap was
active now, but the opposite is actually true.

Let's improve this by saying "Deactivated successfully", which hopefully
clears this up.

(I thought about saying "terminated" or "completed" or "finished" or so
instead, but that too doesn#t make sense if you think about unit types
like swaps or devices.)

src/core/unit.c

@@ -6094,7 +6094,7 @@ void unit_log_success(Unit *u) {
                    "MESSAGE_ID=" SD_MESSAGE_UNIT_SUCCESS_STR,
                    LOG_UNIT_ID(u),
                    LOG_UNIT_INVOCATION_ID(u),
-                   LOG_UNIT_MESSAGE(u, "Succeeded."));
+                   LOG_UNIT_MESSAGE(u, "Deactivated successfully."));
 }
 
 void unit_log_failure(Unit *u, const char *result) {