pgsql: Remove undocumented restriction against duplicate partition key

Remove undocumented restriction against duplicate partition key columns.

transformPartitionSpec rejected duplicate simple partition columns
(e.g., "PARTITION BY RANGE (x,x)") but paid no attention to expression
columns, resulting in inconsistent behavior. Worse, cases like
"PARTITION BY RANGE (x,(x))") were accepted but would then result in
dump/reload failures, since the expression (x) would get simplified
to a plain column later.

There seems no better reason for this restriction than there was for
the one against duplicate included index columns (cf commit 701fd0bbc),
so let's just remove it.

Back-patch to v10 where this code was added.

Report and patch by Yugo Nagata.

Discussion: https://postgr.es/m/[email protected]

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b1af4bcc42f7e95abb9e8bceb894e0ef2acfc55a

Modified Files
--------------
src/backend/commands/tablecmds.c | 15 ---------------
src/test/regress/expected/create_table.out | 5 -----
src/test/regress/sql/create_table.sql | 5 -----
3 files changed, 25 deletions(-)