ANC controller failed to disarm AMASS after runway was opened and elected to permit a landing as the AMASS alarm was activating.

Narrative

Aircraft was on visual approach to Runway 07L. Maintenance 1 and company; performing snow removal; exited Runway 07R and told me that it was now open. Trying to provide a service I asked if the aircraft would prefer RWY 07R because it had just been swept and would be much more convenient to their parking area. They said yes; and I changed their runway and cleared then to land on Runway 07R. I forgot to remove the runway closure X's off the ASDE/AMASS. When the AMASS alarm when off; I immediately went to remove the X's and asked if I had to send the aircraft around. I had not encountered this situation and was unsure what to do. I thought that if I removed the X's; that would fix the problem. I did not get a clear answer from my co-workers. The AMASS stopped alarming and the aircraft landed with the AMASS in normal configuration. After the incident I researched what the appropriate response should have been. I found the reference 3-6-4 in the 7110.65; and now have a clear understanding of what I should have done; regardless of whether or not I thought the runway was clear. Recommendation; there is a check list for opening and closing runways; I should have used that. I could have also written a note to remind myself of the X's on the ASDE/AMASS.