Revert "Namespaceify mtu_expires sysctl"

This reverts commit 7385291d which is
commit 1135fad2 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Idf1067f56f5b98a10198a75bb1ea9d03067980b7
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>