hostname-util: get rid of unused parameter of hostname_cleanup()

All users are now setting lowercase=false.

src/basic/hostname-util.h

@@ -30,7 +30,7 @@ bool hostname_is_set(void);
 char* gethostname_malloc(void);
 
 bool hostname_is_valid(const char *s, bool relax) _pure_;
-char* hostname_cleanup(char *s, bool lowercase);
+char* hostname_cleanup(char *s);
 
 bool is_localhost(const char *hostname);