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.