Remove unzip() SizeHint hack

[ollie27]

This was using an invalid iterator so is likely to end with buggy
behaviour.

It also doesn't even benefit many type in std including Vec so removing it
shouldn't cause any problems.

Fixes: #33468

[rust-highfive]

r? @nikomatsakis

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[alexcrichton]

I believe this was originally used to preallocate space in a Vec, but the implementation of extend since changed to not take advantage of this.

The performance of unzip doesn't seem necessarily that important, so I'd be fine doing so. Curious what others on @rust-lang/libs think though.

[shepmaster]

I think @bluss will be excited to see this go!

[bluss]

Yes! Cool hack, but it breaks the rules of an Iterator impl.

[bluss]

A note from somewhere we discussed this previously: We can do a much better job with specialization anyway.

[brson]

+1

[alexcrichton]

The libs team discussed this PR during triage today and the conclusion was to merge. Thanks again @ollie27!

[alexcrichton]

@bors: r+ 02f9be8

[bors]

⌛ Testing commit 02f9be8 with merge fe96928...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild