Mac system does not provide a forget this Wi-Fi network and if you want to delete old and unused Wi-Fi connections then you use option available as part of Network Preferences.
Step 1: Click the Wi-Fi icon on Mac Status bar and select Open Network Preferences from the drop down list. Another way to access Network Preferences is by clicking Mac icon > System Preferences > Internet and Wireless > Network.
Step 2: Click the Advanced button in Network Preferences window.
Step 3: In the Wi-Fi window, navigate to Preferred Networks and select the networks that have to be deleted. Then click – sign available below the Preferred Networks list.
Step 4: Click OK button on the Wi-Fi window to confirm and save the changes.
If you travel a lot and connect to many Wi-Fi networks then it is recommended to un mark the check box with caption “Remember networks this computer has joined” under Wi-Fi Window. This would ensure that those temporary Wi-Fi networks are not saved on your Mac system.