MDEV-37048 revert MSAN my_vsnprintf_ex for double workaround

5 years ago e843033 worked around a
MSAN bug when retrieving a double from a va_list.

The construct {{__msan_check_mem_is_initialized(ap,size)}} where ap
is a va_list is undefined as {{__msan_check_mem_is_initialized}}
expects a pointer. The implementation detail of va_list is architecture
dependant and on aarch64 this isn't a pointer.

The need to any msan action is no unnecessary since this has been
corrected in recent clang versions.

As such the additions from e843033
have been reverted.

Tested with clang-20.1 and the test cases from MDEV-22690 and
MDEV-22691.

Author: grooverdan

strings/my_vsnprintf.c

@@ -739,13 +739,7 @@ size_t my_vsnprintf_ex(CHARSET_INFO *cs, char *to, size_t n,
     else if (*fmt == 'f' || *fmt == 'g')
     {
       double d;
-#if __has_feature(memory_sanitizer)         /* QQ: MSAN has double trouble? */
-      __msan_check_mem_is_initialized(ap, sizeof(double));
-#endif
       d= va_arg(ap, double);
-#if __has_feature(memory_sanitizer)        /* QQ: MSAN has double trouble? */
-      __msan_unpoison(&d, sizeof(double));
-#endif
       to= process_dbl_arg(to, end, width, d, *fmt);
       continue;
     }