tpsip_connection_dispose: remove assertion that base->self_handle == 0
tpsip_connection_dispose: remove assertion that base->self_handle == 0

This is unnecessary, was never guaranteed, and we'd like to make it not
be the case in a future tp-glib.  See fd.o #17751

freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia