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/conf-parser.c

@@ -515,8 +515,7 @@ int config_parse_many(
                                    #type, rvalue);                      \
                                                                         \
                 return 0;                                               \
-        }                                                               \
-        struct __useless_struct_to_allow_trailing_semicolon__
+        }
 
 DEFINE_PARSER(int, int, safe_atoi);
 DEFINE_PARSER(long, long, safe_atoli);