Removal of currtid()/currtid2() and some table AM cleanup

Hi all,

I have been looking at the ODBC driver and the need for currtid() as
well as currtid2(), and as mentioned already in [1], matching with my
lookup of things, these are actually not needed by the driver as long
as we connect to a server newer than 8.2 able to support RETURNING.  I
am adding in CC of this thread Saito-san and Inoue-san who are the
two main maintainers of the driver for comments.  It is worth noting
that on its latest HEAD the ODBC driver requires libpq from at least
9.2.

I would like to remove those two functions and the surrounding code
for v14, leading to some cleanup:
 6 files changed, 326 deletions(-)

While on it, I have noticed that heap_get_latest_tid() is still
located within heapam.c, but we can just move it within
heapam_handler.c.

Attached are two patches to address both points.  Comments are
welcome.

Thanks,

[1]: https://www.postgresql.org/message-id/20200529005559.jl2gsolomyro4l4n@alap3.anarazel.de
--
Michael