plugins: remove gst_vaapi_plugin_base_get_allowed_srcpad_raw_caps()

plugins: remove gst_vaapi_plugin_base_get_allowed_srcpad_raw_caps()

Since nobody uses it, just remove it.

Thus extract_allowed_surface_formats() is refactored to attend only
gst_vaapi_plugin_base_get_allowed_sinkpad_raw_caps().

Now a surface is created when the image chorma is different from the
previous one. And if the driver has the quirk, it outputs all the
supported image formats without trying them.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/381>