Remove no-longer-needed ExecCountSlots infrastructure.

src/backend/executor/execProcnode.c

src/backend/executor/nodeAgg.c

src/backend/executor/nodeAppend.c

src/backend/executor/nodeBitmapAnd.c

src/backend/executor/nodeBitmapHeapscan.c

src/backend/executor/nodeBitmapIndexscan.c

src/backend/executor/nodeBitmapOr.c

src/backend/executor/nodeCtescan.c

src/backend/executor/nodeFunctionscan.c

src/backend/executor/nodeGroup.c

src/backend/executor/nodeHash.c

src/backend/executor/nodeHashjoin.c

src/backend/executor/nodeIndexscan.c

src/backend/executor/nodeLimit.c

src/backend/executor/nodeMaterial.c

src/backend/executor/nodeMergejoin.c

src/backend/executor/nodeNestloop.c

src/backend/executor/nodeRecursiveunion.c

src/backend/executor/nodeResult.c

src/backend/executor/nodeSeqscan.c