MEDIUM: stconn: remove SE_FL_RXBLK_SHUT

This flag is no more needed, it was only set on shut read to be tested
by cs_rx_blocked() which is now properly tested for shutr as well. The
cs_rx_blk_shut() calls were removed. Interestingly it allowed to remove
a special case in the L7 retry code.

This also ensures we will not risk to set it back on a new endpoint
after cs_reset_endp() via SE_FL_APP_MASK.
5 files changed