Redundant 'enable_eul_grid_forcing_reset' flag in virtual boundary forcing

[sy-cui]

The VirtualBoundaryForcing class currently supports the enable_eul_grid_forcing_reset flag which, if True, set the Eulerian forcing field to zero before transfer the Lagrangian forces to Eulerian grid. This is justified by the Lagrangian-to-Eulerian communication kernel, which performs addition to the Eulerian field instead of replacement. Since this transfer is done exactly once every flow time step, the Eulerian forcing field should always be reset and thus the enable_eul_grid_forcing_reset flag should be True for all cases. Nonetheless, this resetting is invariably enforced in the flow simulator (_navier_stokes_with_forcing_time_step). Therefore, this flag along with the internal zeroing in VirtualBoundaryForcing is redundant.