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 (closed)