alphacombine: Ignore all events coming from the alpha_pad

alphacombine: Ignore all events coming from the alpha_pad

As per usage of this element, everything from this pad is a
duplicate. Instead of implemented needless aggregation, simply
drop all events from this pad and let the one from the main stream
passthrough. Also stop proxying some queries from the alpha pad_too.

This fixes racy test failure:
- validate.file.playback.scrub_forward_seeking.opus_vp9-alpha_webm

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2247>