Since callback signatures are now generated in C++, might as well go
ahead and retain `const` and the pointer type, removing a `const_cast`
in the process.
The expected type of the callback is already known from the AST - this
change reduces dependency on cgo specifics and makes the generated C ABI
entirely cgo-independent - in particular, there is no need to include
`_cgo_export.h` any more.
