Remove an old FIXME comment and inline attribute

[osa1]

Apparently #35870 caused a problem in this code (which originally
returned an impl trait) and #[inline] was added as a workaround, in
ade79d7.

The issue is now fixed and the comment and #[inline] can now be
removed.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[Aaron1011]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit c4b90df with merge 21756682dd6c928ccfaf623929beefa852865f13...

[bors]

☀️ Try build successful - checks-actions
Build commit: 21756682dd6c928ccfaf623929beefa852865f13 (21756682dd6c928ccfaf623929beefa852865f13)

[rust-timer]

Queued 21756682dd6c928ccfaf623929beefa852865f13 with parent 1b1d85f, future comparison URL.

[rust-timer]

Finished benchmarking try commit (21756682dd6c928ccfaf623929beefa852865f13): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit c4b90df has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit c4b90df with merge edeee91...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing edeee91 to master...