MDEV-26443 HAVE_C99_INITIALIZERS is not applicable to C++

Designated initializers were introduced in ISO/IEC 9899:1999 (C99),
but the C code base of MariaDB is supposed to be compatible with the
1990 version of the standard.

The InnoDB code based was switched from C to C++ in
MySQL 5.6 and MariaDB 10.0. C++ did not introduce syntax for
designated initializers until ISO/IEC 14882:2020.
Our C++ code base is still stuck with the 2011 or earlier version of
that standard.

Therefore, this check as well as the macro STRUCT_FLD are best removed.