Fix failure to close listening socket (#3678).

[jmattsson]

Fixes #3678

• This PR is for the dev branch rather than for the release branch.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

The IDF LWIP's netconn_close() no longer returns valid error codes as the #ifdef'd code path the IDF uses does not end up actually doing much, which unfortunately includes actually setting the error code. As such, checking the return code is actively harmful to proper operation for us. The fix here is simply to carry on with the netconn_delete(). This should be safe even if netconn_close() actually errored.

[marcelstoer]

I haven't tested it but the change looks sane given the explanations. Thanks a lot!