In dumb_socketpair(), delete Unix-domain socket path once no longer needed

In dumb_socketpair(), delete Unix-domain socket path once no longer needed

Small follow-up improvement to
https://gitlab.com/openconnect/openconnect/-/merge_requests/320, which made
dumb_socketpair() able to use Unix-domain sockets, on those Windows versions
that support them albeit only with named paths.

This was suggested as a way to prevent the Windows dumb_socketpair()
implementation from leaving behind size-0 files, even if normally only in
temporary directories.

See original suggestion at
https://github.com/microsoft/WSL/issues/4240#issuecomment-1027607891.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>