Cleanup: offsetof instead of my_offsetof

Sys_lc_messages, Sys_lc_time_names: Use offsetof(MY_LOCALE, ...)
because the type has standard layout.

sql/sys_vars.cc

@@ -5425,14 +5425,14 @@ static bool update_locale(sys_var *self, THD* thd, enum_var_type type)
 static Sys_var_struct Sys_lc_messages(
        "lc_messages", "Set the language used for the error messages",
        SESSION_VAR(lc_messages), NO_CMD_LINE,
-       my_offsetof(MY_LOCALE, name), DEFAULT(&my_default_lc_messages),
+       offsetof(MY_LOCALE, name), DEFAULT(&my_default_lc_messages),
        NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(check_locale), ON_UPDATE(update_locale));
 
 static Sys_var_struct Sys_lc_time_names(
        "lc_time_names", "Set the language used for the month "
        "names and the days of the week",
        SESSION_VAR(lc_time_names), NO_CMD_LINE,
-       my_offsetof(MY_LOCALE, name), DEFAULT(&my_default_lc_time_names),
+       offsetof(MY_LOCALE, name), DEFAULT(&my_default_lc_time_names),
        NO_MUTEX_GUARD, IN_BINLOG, ON_CHECK(check_locale));
 
 static Sys_var_tz Sys_time_zone(