Remove u64 support for Poisson #1114

Job	Run time
Check doc	31s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	3m 13s
test (macos-latest, x86_64-apple-darwin, stable)	3m 41s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	17m 57s
test (windows-latest, x86_64-pc-windows-gnu, stable)	4m 22s
test-miri	4m 35s
test-ios	21s
test (windows-latest, x86_64-pc-windows-msvc, beta)	4m 14s
test-no-std	12s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	5m 6s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	4m 48s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	3m 38s
	52m 38s

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