Remove c_char from the PSP module

ef8c0f3 ("Define c_char at top-level...") removes the need for per-OS
definitions of `c_char`. Remove the definition of `c_char` from PEP,
which as not covered in the backport (as this module does not exist on
`main`).

Suggested-by: Taiki Endo <te316e89@gmail.com>

src/psp.rs

@@ -25,7 +25,6 @@ pub type intptr_t = isize;
 pub type uintptr_t = usize;
 pub type ssize_t = isize;
 
-pub type c_char = u8;
 pub type c_long = i64;
 pub type c_ulong = u64;