Revert "tipc: add tipc_bearer_min_mtu to calculate min mtu"

This reverts commit 735c64ea.

It breaks the Android KABI and will be brought back at a later time when
it is safe to do so.

Bug: 161946584
Change-Id: Ibf930d1b18dfb99cf889795d60ab410599434dd7
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>