reuse jsffi in json module (#17165)

* remove unnecessary when statement

* remove outdated codes

* reuse jsffi

* move js json coverage

tests/stdlib/t15835.nim

@@ -1,4 +1,9 @@
-import json
+discard """
+  targets: "c js"
+"""
+
+
+import std/json
 
 type
   Foo = object

tests/stdlib/talgorithm.nim

@@ -1,9 +1,10 @@
 discard """
+  targets: "c js"
   output:'''@["3", "2", "1"]
 '''
 """
 #12928,10456
-import sequtils, algorithm, json
+import std/[sequtils, algorithm, json]
 
 proc test() = 
   try: 

tests/stdlib/tjson_unmarshall.nim

@@ -1,4 +1,5 @@
 discard """
+  targets: "c js"
   output: '''
 Original: (kind: P, pChildren: @[(kind: Text, textStr: "mychild"), (kind: Br)])
 jsonNode: {"kind":"P","pChildren":[{"kind":"Text","textStr":"mychild"},{"kind":"Br"}]}
@@ -6,7 +7,7 @@ Reversed: (kind: P, pChildren: @[(kind: Text, textStr: "mychild"), (kind: Br)])
 '''
 """
 
-import json
+import std/json
 
 type
   ContentNodeKind* = enum