Revert "net: ipv4: Cache pmtu for all packet paths if multipath enabled"

This reverts commit fc07a232 which is
commit 7d3f3b43 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: I5d5cfb6058427ec56c1704182065cd38e2ca192a
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>