Remove a workaround for an old Musl bug

In older versions of Musl, ttyname_r would wrongly return ENOTTY instead
of EBADF.  We expected that bug in our test suite.  But Rust
1.37.0 updated Musl to 1.1.22, which fixes the problem.  This change
reverts our workaround.

https://www.openwall.com/lists/musl/2018/09/15/2
rust-lang/rust@aaf6698

Author: asomers

test/test_unistd.rs

@@ -1064,13 +1064,7 @@ fn test_ttyname_not_pty() {
 }
 
 #[test]
-#[cfg(all(not(target_os = "redox"), not(target_env = "musl")))]
+#[cfg(all(not(target_os = "redox")))]
 fn test_ttyname_invalid_fd() {
     assert_eq!(ttyname(-1), Err(Error::Sys(Errno::EBADF)));
 }
-
-#[test]
-#[cfg(all(not(target_os = "redox"), target_env = "musl"))]
-fn test_ttyname_invalid_fd() {
-    assert_eq!(ttyname(-1), Err(Error::Sys(Errno::ENOTTY)));
-}