rustdoc: Don't link to stable/beta docs for primitives

[jyn514]

This has two issues:

1. The test suite is not set up for links that differ based on the
   channel. This is not a giant deal; it's fixed by
   jyn514@2a21264.
2. It's inconsistent with non-primitive types, because libstd uses
   html_root_url = "rust-lang.org/nightly". Fixing this is non-trivial,
   and the usefulness has also been called into question; see
   https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Rustdoc.20unconditionally.20links.20to.20nightly.20libstd.20docs.

To avoid being inconsistent, this reverts linking to beta for now.

cc @pietroalbini, this should fix the test failure in #84909 (comment) if you backport it.

[rust-highfive]

r? @ollie27

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[jyn514]

hmm, r? @Mark-Simulacrum maybe

[pietroalbini]

@bors r+

@rustbot modify-labels: beta-accepted

[bors]

📌 Commit 19d4b04 has been approved by pietroalbini

[pietroalbini]

@bors r-

The change also breaks Rustdoc UI tests, see #84909 (comment):

30    |         ^^^^^^^^^^^^
31    = note: `#[warn(rustdoc::missing_crate_level_docs)]` implied by `#[warn(rustdoc::all)]`
32    = help: The following guide may be of use:
-            https://doc.rust-lang.org/nightly/rustdoc/how-to-write-documentation.html
+            https://doc.rust-lang.org/beta/rustdoc/how-to-write-documentation.html
35 warning: missing code example in this documentation
36   --> $DIR/check.rs:4:1

I'll open a PR to revert the whole 3478f83 commit.

[pietroalbini]

Superseded by #84950.