remove test for `get_env` `noexcept`-ness from `inline_scheduler`

[ericniebler]

[exec.inline.scheduler]/p3.2 reads

• 3.2 the expression get_completion_scheduler<set_value_t>(get_env(sndr)) has type​ inline_scheduler and is potentially-throwing if and only if get_env(sndr) is potentially-throwing.

get_env(sndr) is never potentially-throwing because it mandates that sndr.get_env() cannot throw. See [exec.get.env].

also, get_completion_scheduler<set_*_t>(attrs) mandates that attrs.query(get_completion_scheduler<set_*_t>) cannot throw.

Proposed resolution

Change [exec.inline.scheduler]/p3.2 as follows:

• 3.2 the expression get_completion_scheduler<set_value_t>(get_env(sndr)) has type​ inline_scheduler and is potentially-throwing if and only if get_env(sndr) is potentially-throwing.

[ericniebler]

now cplusplus/LWG#4471: https://cplusplus.github.io/LWG/issue4471