Revert "Namespaceify min_pmtu sysctl"

This reverts commit 68679217 which is
commit 1de6b15a 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: I53d70488adf6968cde4fad9d2b327bb1ef3cbc12
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>