You can remove the conditions by extracting the first element of the ConditionalExpression:
Solve[ForAll[{x, y}, 
    Equivalent[
     m (x/a) + n (y/b) == 
      1, ((t1 - t2) (b (-1 + t1 t2) x + 
           a (b + b t1 t2 - (t1 + t2) y)))/((1 + t1^2) (1 + t2^2)) == 
      0]], {m, n}, Reals] /. 
  ConditionalExpression -> (#1 &) // Simplify