gsignal: Drop unnecessary warnings from g_signal_lookup()
gsignal: Drop unnecessary warnings from g_signal_lookup()

The `G_TYPE_IS_INSTANTIATABLE()` check is redundant with a precondition
on the function.

The `g_type_class_peek()` check seems like a pointless restriction: it
should be possible to check for a signal from a class init function.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #650

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib