opt/bench: fix up recently added query to EndToEnd benchmark

For some reason, the JSON parser is not happy when we're using the
placeholders for the JSON type (due to using escape characters somewhere
in the driver), so this commit inlines the placeholder values into the
query (we already do this for a couple of other queries). Additionally,
we forgot to perform the cleanup to avoid duplicate key violations.

Release note: None

pkg/sql/opt/bench/bench_test.go

@@ -455,9 +455,10 @@ var queries = [...]benchQuery{
 		args:  []interface{}{"'abc'"},
 	},
 	{
-		name:  "json-insert",
-		query: "INSERT INTO json_table(k, i, j) VALUES ($1, $2, $3)",
-		args:  []interface{}{1, 10, `'{"a": "foo", "b": "bar", "c": [2, 3, "baz", true, false, null]}'`},
+		name:    "json-insert",
+		query:   `INSERT INTO json_table(k, i, j) VALUES (1, 10, '{"a": "foo", "b": "bar", "c": [2, 3, "baz", true, false, null]}')`,
+		args:    []interface{}{},
+		cleanup: "TRUNCATE TABLE json_table",
 	},
 	{
 		name: "batch-insert-one",