Revert "tipc: add tipc_bearer_min_mtu to calculate min mtu"

This reverts commit 097ea78d which is
commit 3ae6d66b upstream.

It breaks the Android kernel ABI and is not needed for Android devices,
so it is safe to revert for now.  If it is determined that it is needed
in the future, it can be brought back in an abi-preserving way.

Bug: 161946584
Change-Id: I0b32b3b8d7c81895dd1680f7aec65d38cace080d
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>