sctp: Remove useless last_time_used variable

The transport last_time_used variable is rather useless.
It was only used when determining if CWND needs to be updated
due to idle transport.  However, idle transport detection was
based on a Heartbeat timer and last_time_used was not incremented
when sending Heartbeats.  As a result the check for cwnd reduction
was always true.  We can get rid of the variable and just base
our cwnd manipulation on the HB timer (like the code comment sais).
We also have to call into the cwnd manipulation function regardless
of whether HBs are enabled or not.  That way we will detect idle
transports if the user has disabled Heartbeats.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>