Remove option to build without USE_SOCKETS_AS_HANDLES on Windows

Remove option to build without USE_SOCKETS_AS_HANDLES on Windows

The option is always defined by default and can't be disabled from the
makefiles. Manually disabling it causes several tests to fail, which
nobody has reported, so we presume nobody does this. The non-default
configuration is believed to be historical cruft with no value now, and
has clearly bitrotted in recent years (hence the test failures), so
remove it to simplify the codebase slightly.