Removing unused wrapper to libc::close.

It's not used and it passes the test after removing.
And some function inside rust repository reimplement it in severals ways like in: 
`src/libnative/io/helper_thread.rs`:
```rust
169:    pub fn close(fd: libc::c_int) {
170-        let _fd = FileDesc::new(fd, true);
171-    }
172-}
```

Or directly use `libc::CloseHandler`
`src/libnative/io/helper_thread.rs`:
```rust
194:    pub fn close(handle: HANDLE) {
195-        assert!(unsafe { CloseHandle(handle) != 0 });
196-    }
```
Or `libc::close`:
`src/librustdoc/flock.rs`:
```rust
108-            if ret == -1 {
109:                unsafe { libc::close(fd); }
110-                fail!("could not lock `{}`", p.display())
111-            }
```