pgsql: Remove unnecessary code in dependency_is_compatible_expression()

Remove unnecessary code in dependency_is_compatible_expression().

Scanning the expression for compatible Vars isn't really necessary,
because the subsequent match against StatisticExtInfo entries will
eliminate expressions containing other Vars just fine.  Moreover,
this code hadn't stopped to think about what to do with
PlaceHolderVars or Aggrefs in the clause; and at least for the PHV
case, that demonstrably leads to failures.  Rather than work out
whether it's reasonable to ignore those, let's just remove the
whole stanza.

Per report from Richard Guo.  Back-patch to v14 where this code
was added.

Discussion: https://postgr.es/m/CAMbWs48Mmvm-acGevXuwpB=g5JMqVSL6i9z5UaJyLGJqa-XPAA@mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7c509f7e5a817ea676479d33bb975d72785ff106

Modified Files
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src/backend/statistics/dependencies.c | 28 +++-------------------------
1 file changed, 3 insertions(+), 25 deletions(-)