get_post_custom() should use use get_metadata()

[scribu]

I found out that get_post_custom() is equivalent to calling get_metadata() without a meta key, so we should use that.

almost equivalent

Related: #17013

Patch still applies cleanly.

Hmm, get_metadata() contains much more and different lines of code as what you remove with the proposed patch.

You've got it backwards: I'm replacing get_post_meta() with get_metadata(), not the other way around.

Of course get_metadata() contains much more code, since it's a more general function.

I doubt this got the testing or considerations required here. First patch was marked for commit but it wouldn't have worked when ID = 0. As said in IRC, seems fine but not really a necessary change at this time. Punting.

#19708