Revert #1295

[msimberg]

The PR passed pika's tests, but leads to uses of empty any_senders in DLA-Future. It's still unclear if the mistake is in DLA-Future or pika. Reverting the change in let_value is enough to make DLA-Future's tests pass again, but reverting the whole PR until I've investigated it further (other algorithms may have issues that we don't trigger in DLA-Future).

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.03% (target: -1.00%)	✅ 94.12% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2df06f7)	18285	13814	75.55%
Head commit (36f287a)	18272 (-13)	13798 (-16)	75.51% (-0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1334)	51	48	94.12%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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