Assume HPX_HAVE_STDEXEC

[isidorostsa]

This PR is to remove all places in the source code that branch on the existence of stdexec. This PR is auto-generated, so we could apply it at any time. Right now, CI is failing because the builders have not been configured to assume C++20 yet.

[isidorostsa]

This was done using the unifdef tool (possibly helpful script: https://gist.github.com/isidorostsa/1f334e240279b2664ff37d645f4c145a)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 24621cb1	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (24621cb)	Report Missing	Report Missing	Report Missing
Head commit (fb679f8)	5656	0	0.00%

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 (#6708)	0	0	∅ (not applicable)

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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Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩