fix incorrect overload of join() function

https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/evaluate-expressions-in-workflows-and-actions#join

Author: rhysd

expr_sema.go

@@ -142,14 +142,6 @@ var BuiltinFuncSignatures = map[string][]*FuncSignature{
 				StringType{},
 			},
 		},
-		{
-			Name: "join",
-			Ret:  StringType{},
-			Params: []ExprType{
-				StringType{},
-				StringType{},
-			},
-		},
 		// When the second parameter is omitted, values are concatenated with ','.
 		{
 			Name: "join",
@@ -158,13 +150,6 @@ var BuiltinFuncSignatures = map[string][]*FuncSignature{
 				&ArrayType{Elem: StringType{}},
 			},
 		},
-		{
-			Name: "join",
-			Ret:  StringType{},
-			Params: []ExprType{
-				StringType{},
-			},
-		},
 	},
 	"tojson": {{
 		Name: "toJSON",