Remove HeapTupleheaderSetXmin{Committed,Invalid} functions

Hi,

When I am reading the code, I first thought I can do something in
HeapTupleheaderSetXminCommitted, then I realized we have SetHintBits.
After some research, I find HeapTupleHeaderSetXminCommitted is never
used and it looks not safe to use after comparing with SetHintBits. So
to avoid future confusion or misuse, I'd suggest to remove it. I think
HeapTupleHeaderSetXminInvalid should be the same. So here is the patch.

commit ed905d64c6b81a78627f895918d4ac723d46865c (HEAD -> heap_set_bits)
Author: Andy Fan <zhihuifan1213(at)163(dot)com>
Date: Wed Jun 25 07:35:32 2025 +0000

Remove HeapTupleheaderSetXminCommitted/Invalid functions

They are neither be used nor be safe to be used, User should use
SetHintBits instead.

What do you think?

--
Best Regards
Andy Fan