[Clang] [NFC] Remove default argument in ASTUnit.h (#78566)

This removes a default argument that is currently broken in C++23 mode
due to `std::default_delete` now being `constexpr`. This is a known
problem (see #74963, #59966, #69996, and a couple more), fixing which
will probably take some time, so this at least makes it possible to
compile `ASTUnit.h` in C++23 mode.

Note that we can’t simply include the header that provides the
definition of the class causing the problem either, as that would create
a circular dependency.