remove `Function::coeffs_for_jun` #1922

Job	Run time
macos-latest: clang++ Release Threads	10m 4s
macos-latest: clang++ Release OneTBB	1m 20s
macos-latest: clang++ Release PaRSEC	2m 4s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	23m 5s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	1m 44s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	1m 48s
macos-latest: clang++ Debug Threads	19m 17s
macos-latest: clang++ Debug OneTBB	2m 24s
macos-latest: clang++ Debug PaRSEC	2m 21s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	48m 20s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	1m 50s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	1m 51s
	1h 56m 8s

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