Fix g_source_remove bug of NemoDesktopLinkMonitor

The function `g_source_remove' is used to remove GSource ids, not signal
handlers. To unlink handlers by id, `g_signal_handler_disconnect' has to
be used instead. However, handlers connected via
`g_signal_connect_object' get disconnected automatically if the object
that is supposed to be passed to the handler after signal emissions gets
destroyed. Unhooking the handlers in NemoDesktopLinkMonitor's finalize
function is therefore unnecessary.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>