openbsd: remove unused openbsd code

This code is in a block (libc::consts::os) that openbsd don't include
This one is for freebsd and dragonfly. There is another block for openbsd below.

Remove the unneed declaration.

src/liblibc/lib.rs

@@ -3753,9 +3753,6 @@ pub mod consts {
             #[cfg(target_os = "dragonfly")]
             pub const PTHREAD_STACK_MIN: size_t = 1024;
 
-            #[cfg(target_os = "openbsd")]
-            pub const PTHREAD_STACK_MIN: size_t = 2048;
-
             pub const CLOCK_REALTIME: c_int = 0;
             pub const CLOCK_MONOTONIC: c_int = 4;
         }