syscall/freebsd: remove Sendfile from unimplemented list

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6343084
diff --git a/src/pkg/syscall/syscall_freebsd.go b/src/pkg/syscall/syscall_freebsd.go
index 903e5b6..147f552 100644
--- a/src/pkg/syscall/syscall_freebsd.go
+++ b/src/pkg/syscall/syscall_freebsd.go
@@ -326,7 +326,6 @@
 // __pthread_canceled
 // __semwait_signal
 // Proc_info
-// Sendfile
 // Stat64_extended
 // Lstat64_extended
 // Fstat64_extended