pgsql: Simplify productions for FORMAT JSON [ ENCODING name ]

Simplify productions for FORMAT JSON [ ENCODING name ]

This removes the production json_encoding_clause_opt, instead merging
it into json_format_clause.  Also remove the auxiliary
makeJsonEncoding() function.

Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/202312071841.u2gueb5dsrbk%40alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d3fe6e90bab52b8cbf085cfad5e4da110a2cd373

Modified Files
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src/backend/nodes/makefuncs.c | 21 -------------------
src/backend/parser/gram.y     | 49 +++++++++++++++++++++++++++++--------------
src/include/nodes/makefuncs.h |  1 -
3 files changed, 33 insertions(+), 38 deletions(-)