core: drop unnecessary __useless_struct_to_allow_trailing_semicolon__

ISO C does not allow empty statements outside of functions, and gcc
will warn the trailing semicolons when compiling with -pedantic:

  warning: ISO C does not allow extra ‘;’ outside of a function [-Wpedantic]

But our code cannot compile with -pedantic anyway, at least because

  warning: ISO C does not support ‘__PRETTY_FUNCTION__’ predefined identifier [-Wpedantic]

Without -pedatnic, clang and even old gcc (3.4) generate no warnings about
those semicolons, so let's just drop __useless_struct_to_allow_trailing_semicolon__.

src/shared/bus-util.h

@@ -150,8 +150,7 @@ int bus_log_create_error(int r);
                         return r;                                       \
                                                                         \
                 return 1;                                               \
-        }                                                               \
-        struct __useless_struct_to_allow_trailing_semicolon__
+        }
 
 #define BUS_PROPERTY_DUAL_TIMESTAMP(name, offset, flags) \
         SD_BUS_PROPERTY(name, "t", bus_property_get_usec, (offset) + offsetof(struct dual_timestamp, realtime), (flags)), \