Remove <sys/mtio.h> workaround.

Bug: N/A
Test: builds
Change-Id: I4f2f615720639cb6ec4ac8a6b08c5907008f98ee
diff --git a/toolbox/upstream-netbsd/include/sys/mtio.h b/toolbox/upstream-netbsd/include/sys/mtio.h
deleted file mode 100644
index 8fb5655..0000000
--- a/toolbox/upstream-netbsd/include/sys/mtio.h
+++ /dev/null
@@ -1 +0,0 @@
-#include <linux/mtio.h>