How to remove iPhone passcode, if I don't have the iPhone anymore?

If you want to eliminate the passcode, you need to erase the phone before removing it from your account.

1. In Find My iPhone on iCloud.com, click All Devices, then select the device you want to erase.
2. If you don’t see All Devices, it’s because you’ve already selected a device. Click the name of the current device in the center of the toolbar to access the Devices list, then select a new device.
3. Erase the device.
4. Because the device isn’t lost, don’t enter a phone number or message.
5. If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.
6. When the device is erased, click Remove from Account.
7. 
8. All your content is erased, and someone else can now activate the device.

The iPhone is activation locked and someone else wants to use it?

See this:

1. Go to www.iCloud.com/find.
2. Sign in with your Apple ID and password.
3. At the top, click All Devices.
4. Select the device that you want to remove from iCloud.
5. Click Remove from Account