[MPM] Cleanup elements

[ncrescenzio]

🆕 Changelog

• Remove local flags that were not used (COMPUTE_RHS_VECTOR, COMPUTE_LHS_MATRIX, COMPUTE_RHS_VECTOR_WITH_COMPONENTS, COMPUTE_LHS_VECTOR_WITH_COMPONENTS)
• Remove variables aux_MP_pressure, aux_MP_velocity and aux_MP_displacement from MPMUpdatedLagrangianUP (these variables always have value zero)
• Remove variable mFinalizedStep
• Use AtomicAdd instead of SetLock/SetUnlock
• Use #pragma once