pgsql: ExecModifyTable: use context.planSlot instead of planSlot

ExecModifyTable: use context.planSlot instead of planSlot

There's no reason to keep a separate local variable when we have a place
for it elsewhere.  This allows to simplify some code.

Reviewed-by: Michaël Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/202204191345.qerjy3kxi3eb@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3dcc6bf4068a29be2bebee80bb919f8057af0fd9

Modified Files
--------------
src/backend/executor/nodeModifyTable.c | 24 ++++++++++--------------
1 file changed, 10 insertions(+), 14 deletions(-)