Remove specialization from iter::Fuse

[cuviper]

Fixes #85863 by removing Fuse's specialization altogether.

[rust-highfive]

r? @kennytm

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[cuviper]

Queuing for a performance run, especially relative to #86765.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit dc49dd0 with merge 3685c31a52d407c58e576c060e96a12744df78f8...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3685c31a52d407c58e576c060e96a12744df78f8 (3685c31a52d407c58e576c060e96a12744df78f8)

[rust-timer]

Queued 3685c31a52d407c58e576c060e96a12744df78f8 with parent 868c702, future comparison URL.

[rust-timer]

Finished benchmarking try commit (3685c31a52d407c58e576c060e96a12744df78f8): comparison url.

Summary: This change led to significant regressions 😿 in compiler performance.

• Moderate regression in instruction counts (up to 1.1% on incr-patched: b9b3e592dd cherry picked builds of style-servo-debug)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

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 +perf-regression

[cuviper]

Closing in favor of #86765.