Remove thread_reduction_factor #1502

Job	Run time
clang-format	50s
install-and-build (USE_MUMPS=false)	2m 54s
install-and-build (USE_MUMPS=true)	5m 42s
run-unit-test (true)	3m 0s
run-unit-test (false)	2m 38s
codecov	10s
	15m 14s

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