Revert "tipc: add tipc_bearer_min_mtu to calculate min mtu"

This reverts commit 5cf99d5f which is
commit 3ae6d66b upstream.

It breaks the Android kernel abi, so revert it for now.  If it is needed
in the future, it can be brought back in an ABI-safe way.

Bug: 161946584
Change-Id: I2ef502e5e126be33ac37cf124e7bfc7a2e7098f2
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>