Update MFEM

[chapman39]

The purpose of updating MFEM is to to enable level set shape optimization.

• Updates mfem to current master at the time of writing
• Removes mfem patch (as it's now in current mfem) Fixes #1185

[chapman39]

Failed test on azure: https://dev.azure.com/llnl-serac/serac/_build/results?buildId=12324&view=logs&j=8418a147-cefe-5fbf-eb11-e069d1299bbe&t=02db9b20-7613-5b66-5602-b48459f634fd&l=55719

41: /home/serac/serac/src/serac/physics/tests/dynamic_solid_adjoint.cpp:279: Failure
41: The difference between directional_deriv and (qoi_plus - qoi_base) / eps is 7.6006933847041342e+24, which exceeds 16 * eps, where
41: directional_deriv evaluates to 7.6006933847041342e+24,
41: (qoi_plus - qoi_base) / eps evaluates to 0.091512106781266311, and
41: 16 * eps evaluates to 3.1999999999999999e-06.
41: 
41: [  FAILED  ] SolidMechanicsSensitivityFixture.InitialVelocitySensitivities (401 ms)

I'll try a re-run