[libc] Remove superfluous explicitly defaulted constructors (#177471)

Any user-defined constructor, even an `= default` one, is
incompatible with aggregate initialization in C++20.

The default constructor does not need to be declared at all in
these cases.  It's implicit.

GitOrigin-RevId: 8735e0c5145d9392b85edd54ca17481eb468d4bf