pgsql: Remove gen_node_support.pl's special treatment of EquivalenceCla

Remove gen_node_support.pl's special treatment of EquivalenceClasses.

It seems better to deal with this by explicit annotations on the
fields in question, instead of magic knowledge embedded in the
script.  While that creates a risk-of-omission from failing to
annotate fields, the preceding commit should catch any such
oversights.

Discussion: https://postgr.es/m/263413.1669513145@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4c689a69eef639caa881539ee546ff1a5b11f98f

Modified Files
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src/backend/nodes/gen_node_support.pl | 40 +++++++++++++++++++++++++++++------
src/include/nodes/nodes.h             |  6 ++++++
src/include/nodes/pathnodes.h         | 17 +++++++++------
3 files changed, 49 insertions(+), 14 deletions(-)