Remove duplicate R3bar accumulation line that doubles bubble perturbation

Character-for-character identical duplicate line causes R3bar to
always be 2x the intended value for QBMM cases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/pre_process/m_assign_variables.fpp

@@ -231,7 +231,6 @@ contains
         if (qbmm) then
             do i = 1, nb
                 R3bar = R3bar + weight(i)*0.5_wp*(q_prim_vf(bubxb + 1 + (i - 1)*nmom)%sf(j, k, l))**3._wp
-                R3bar = R3bar + weight(i)*0.5_wp*(q_prim_vf(bubxb + 1 + (i - 1)*nmom)%sf(j, k, l))**3._wp
             end do
         else
             do i = 1, nb