net: tcp: Remove trigger of send_data_timer when window full

Likely this trigger of the send_data_timer was an alternative for the
function that has been filled in by the ZWP transmission.
At the moment this timer has the potential to cause spurious
retransmissions that can degrade the throughput of the network stack.
Second to that it can accelerate the retransmission process, quickly
running to the number of retransmissions, causing a connection failure.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>