remove det.cpp include in mfem

[rfhaque]

This PR removes the explicit copy of the missing include in mfem that used by remhos.
Fixes #852, follow up to #770

This PR can be merged once

• Decompose det.cpp into separate source and header files mfem/mfem#4866 is merged
• Change mfem include header name CEED/Remhos#53 is merged

Adding/modifying a benchmark (docs: Adding a Benchmark)

• If package.py upstreamed to Spack is insufficient, add/modify repo/benchmark_name/package.py plus: create, self-assign, and link here a follow up issue with a link to the PR in the Spack repo.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.40%. Comparing base (cae351f) to head (f5d2757).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #871   +/-   ##
========================================
  Coverage    41.40%   41.40%           
========================================
  Files           22       22           
  Lines         1512     1512           
  Branches       214      214           
========================================
  Hits           626      626           
  Misses         841      841           
  Partials        45       45           

Flag	Coverage Δ
unittests	41.40% <ø> (ø)

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