Make get_completion_signatures align with the standard

[yulin25]

According to the standard, STDEXEC::get_completion_signatures shall have only one constraint: environment count <= 1.
When the customization does not exist or the member get_completion_signatures customization is ill‑formed upon instantiation, STDEXEC::get_completion_signatures should throw an exception, while the invocation of STDEXEC::get_completion_signatures remains well-formed.
The internal STDEXEC::__compsigs::__get_completion_signatures_helper already does what the standard says, but STDEXEC::get_completion_signatures adds additional constraints, which does not align with the standard.
This PR removes those additional constraints.

[copy-pr-bot]

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