tcp_cmds: Remove README

It contained outdated info about $RHOST, which is *not* mandatory any more
because all tests has been converted to tst_net.sh. Besides that there
is a more up to date doc in testcases/network/README.md.

Also $PASSWD is not used any more since ftp01.sh removal (the last test
which was using it).

Follow-up: b8531de ("tcp_cmds: Remove ftp01.sh test, binary data files")
Signed-off-by: Petr Vorel <pvorel@suse.cz>

Author: pevik