pgsql: Revert commit 928e755d22.

Revert commit 928e755d22.

This commit reverts the fix "Make inherited TRUNCATE perform access
permission checks on parent table only" only in the back branches.

It's not hard to imagine that there are some applications expecting
the old behavior and the fix breaks their security. To avoid this
compatibility problem, we decided to apply the fix only in HEAD and
revert it in all supported back branches.

Discussion: https://postgr.es/m/21015.1580400165@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c15b17f9276ef9a870775c7ed5e53ab4c91b2444

Modified Files
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src/backend/commands/tablecmds.c         | 67 +++++++++-----------------------
src/test/regress/expected/privileges.out | 21 ----------
src/test/regress/sql/privileges.sql      | 14 -------
3 files changed, 18 insertions(+), 84 deletions(-)