xz: Remove an outdated NetBSD-specific comment.

Nowadays errno == EFTYPE is documented in open(2).
diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index e0f44d0..6598ef6 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -401,10 +401,6 @@
 			was_symlink = true;
 
 #	elif defined(__NetBSD__)
-		// As of 2010-09-05, NetBSD doesn't document what errno is
-		// used with O_NOFOLLOW. It is EFTYPE though, and I
-		// understood that is very unlikely to change even though
-		// it is undocumented.
 		if (errno == EFTYPE)
 			was_symlink = true;