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