net: tcp: Revert: Do not accept new data in retransmission mode

After the window_full function has been fixed by looking at the
send_data_total instead of the unacked_len. There is no risk
in sending data in transmission mode.

This reverts commit 0088aaef.

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