Revert "README.dm: Update timely-dataflow link to the current repository." #711

Job	Run time
lint	2m 24s
check-minimal-versions	1m 4s
check (macos-11.0)	1s
test (7.7.0, 3.7)	2m 33s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	4m 15s
test (7.5.1, 3.6)	2m 39s
check (ubuntu-20.04, tracing)	1m 59s
test (7.5.1, 3.5)	1m 57s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	4m 50s
	21m 42s

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