Can I erase an apple watch to remove its iCloud lock so that I can pair it with a new iphone

You can but you do need the iCloud / Apple ID account. See: How to pair your Apple Watch with a new iPhone - Apple Support

You can ask apple to remove the activation lock online on this page;

Remove the Activation Lock Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.