Fix default-port regression in erc-select-read-args

Fix default-port regression in erc-select-read-args

* lisp/erc/erc.el (erc--warn-unencrypted): New function, likely
temporary, to warn new users connecting interactively to the default
server, "irc.libara.chat", via the default non-TLS port, 6667.
(erc-select-read-args): Remove stray code from incomplete feature
introduced by bug#56514.  Ensure connecting always works with default
port, which is non-TLS.  Respect `erc-prompt-for-password' when user
pastes URL containing password component into "server" prompt.  Maybe
add `erc--warn-unencrypted' as one-off hook for impending connection.
* test/lisp/erc/erc-tests.el (erc-select-read-args): Always expect
password prompt and sometimes a non-TLS port when `erc' called
interactively.  (Bug#60428.)