unix: remove *xattr from list of unimplemented syscalls on FreeBSD

These are all implemented on FreeBSD.

Change-Id: I5f3558c19d7540e08cfa77a06fffd80a9c80f1f3
Reviewed-on: https://go-review.googlesource.com/128175
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_freebsd.go

@@ -602,14 +602,6 @@ func Uname(uname *Utsname) error {
 // Watchevent
 // Waitevent
 // Modwatch
-// Getxattr
-// Fgetxattr
-// Setxattr
-// Fsetxattr
-// Removexattr
-// Fremovexattr
-// Listxattr
-// Flistxattr
 // Fsctl
 // Initgroups
 // Posix_spawn