Depends on the amount of space you have and your lifestyle.

If it's for resale, consider who the likely buyers would be (ask a local Realtor if don't know). If it will be a single professional for example, you might get away without.

It's already plumbed so why not leave it? I've never heard of anyone saying they don't want a bathroom with 2 sinks, but there's plenty of people who don't want a bath with only one.

In today's world, most people expect to have two sinks in a master bath. That said, it also depends on the size of the vanity and overall price point of the home.