Remove gen_node_support.pl's ad-hoc ABI stability check.

Remove gen_node_support.pl's ad-hoc ABI stability check.

We installed this in commit eea9fa9b2 to protect against foreseeable
mistakes that would break ABI in stable branches by renumbering
NodeTag enum entries.  However, we now have much more thorough
ABI stability checks thanks to buildfarm members using libabigail
(see the .abi-compliance-history mechanism).  So this incomplete,
single-purpose check seems like an anachronism.  I wouldn't object
to keeping it were it not that it requires an additional manual step
when making a new stable git branch.  That seems like something easy
to screw up, so let's get rid of it.

This patch just removes the logic that checks for changes in the last
auto-assigned NodeTag value.  We still need eea9fa9b2's cross-check
on the supplied list of header files, to prevent divergence between
the makefile and meson build systems.  We'll also sometimes need the
nodetag_number() infrastructure for hand-assigning new NodeTags in
stable branches.

Discussion: https://postgr.es/m/1458883.1776143073@sss.pgh.pa.us