ghostpad: Deprecate gst_ghost_pad_construct()

ghostpad: Deprecate gst_ghost_pad_construct()

Instead do everything it did as part of GObject::constructed() and
change the function to always return TRUE.

gst_ghost_pad_construct() was meant to be called by subclasses right
after construction of the object to finish construction as it can fail
in theory. In practice it's impossible for it to fail, even more so if
called directly from GObject::constructed(): The only failure condition
is if the newly created proxy pad already has a parent, which is
impossible at this point as nothing else can have a reference to it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/540>