How to drop index from mongodb schema using mongoose?

I'm trying to remove an index from my mongoDB collection in node.js application using mongoose. I tried using model.collection.dropIndex("username") but it gives me an error UnhandledPromiseRejectionWarning: MongoError: index not found with name [username].

Here is my Schema

var mongoose = require("mongoose"),
  Schema = mongoose.Schema;

var userTable = new Schema({
  firstname: { type: String, required: true },
  lastname: { type: String, required: true },
  username: { type: String },
  salt: { type: String },
  passwordHash: { type: String },
  email: { type: String, unique: true, required: true },
  sessionToken: { type: String },
  dateCreated: { type: String, default: new Date().toString() },
  loginHistory: [String]
});

module.exports = mongoose.model("userTable", userTable);

When I perform the query in mongo shell from the terminal using command db.usertable.find({}), I can see that the results still have username field. I also tried after removing the username field from schema file, but even that didn't help.

Thanks in advance.

This drops all the indexes of the collection except for the object id

db.collection.dropIndexs();

to delete a certain index

first type the command

 db.collecction.getIndexes();

You will see something like above the in the red square is the index name .

db.collection.dropIndex( { "indexname": 1 } )