Remove unnecessary call to DuplicateHandle() in Win32 builds of sbbs_t::init()

The client_socket_dup is overwritten after with the return value of accept(),
so this was likely a socket/handle leak (since commit 74470573, 17 years ago).

Noticed this only after logging a new error in an SSH connection when this
call to DuplicateHandle() failed for some reason (WinError 6), but it seems
like it's been redundant code for a long time now. So just ace it and possibly
fix a resource leak.