Removing a wall underneath joists that are already attached to beam in attic
Often we’ll design beams into our buildings for “future” use. Your house may have one of these, but before you remove the wall and rely on the beam, you’ll need to investigate: 1) actual loads on the joists, 2) any independent loads on beam, 3) size of support posts at each end of beam, 4) size of “new” footing under each support post at each end of beam. 

1) If your roof joists are 2x6’s spanning 11’ and 13’, I’ll assume you have little or no snow load. Therefore, the minimum roof load would be about 40 lbs. per square foot on the 2x6’s, which translates to 480 lbs. per square foot on the beam. Therefore, 2 - 2x8’s maximum span is about 8’, depending on the species and grade of the 2x8’s. Or, if you increase the beam to 3 - 2x8’s then the allowable span would be about 11’. 

2) Before you determine the maximum allowed opening, you’ll need to verify there are no other loads, like a roof beam resting on the beam. 

3) The beam will need to be “fully” supported at each end. That means a minimum of 2 - 2x studs (or a 4x4 post) will be required at each end. 

4) If the original location had designed the beam, they probably designed an adequate footing under each end to transfer the load to the soil. However, I doubt they knew about the beam when they moved the house and therefore there isn’t a footing under each post. 

Moving a house and placing it on a new foundation requires a Building Permit. Maybe you can contact your local Building Codes Department and look at the submitted plans.