MDEV-22690 MSAN use-of-uninitialized-value in optimizer_trace

This was actually fixed in commit e843033d
and all that we need to do is to remove an unnecessary work-around.

sql/my_json_writer.cc

@@ -180,9 +180,6 @@ void Json_writer::add_size(longlong val)
 void Json_writer::add_double(double val)
 {
   char buf[64];
-#if __has_feature(memory_sanitizer)  // FIXME: remove this workaround for
-  __msan_unpoison(&val, sizeof val); // main.range_mrr_icp & many other tests
-#endif
   size_t len= my_snprintf(buf, sizeof(buf), "%-.11lg", val);
   add_unquoted_str(buf, len);
 }