Prepare for removal of compiler plugin support.

[nnethercote]

rust-lang/rust#116412 will remove support for compiler plugins from rustc, which includes the entry in The Rust Unstable Book. This commit removes a link to that entry so it won't be broken when that PR merges.

[nnethercote]

This is my first time contributing to the book, so I don't know if I'm doing it right.

I'm making this change because without it, the rustc tests fail with the following output:

Building stage0 tool lint-docs (x86_64-unknown-linux-gnu)
   Compiling rustix v0.38.19
   Compiling serde v1.0.185
   Compiling tempfile v3.8.0
   Compiling serde_json v1.0.99
   Compiling lint-docs v0.1.0 (/home/njn/dev/rust2/src/tools/lint-docs)
    Finished release [optimized + debuginfo] target(s) in 4.52s
Running stage1 lint-docs (x86_64-unknown-linux-gnu)
Rustbook (x86_64-unknown-linux-gnu) - rustc
Rustbook (x86_64-unknown-linux-gnu) - cargo
Rustbook (x86_64-unknown-linux-gnu) - clippy
Rustbook (x86_64-unknown-linux-gnu) - embedded-book
Rustbook (x86_64-unknown-linux-gnu) - edition-guide
Rustbook (x86_64-unknown-linux-gnu) - style-guide
Building stage0 tool linkchecker (x86_64-unknown-linux-gnu)
    Finished release [optimized + debuginfo] target(s) in 0.14s
Testing stage0 Linkcheck (x86_64-unknown-linux-gnu)
book/compiler-plugins.html:37: broken link - `unstable-book/language-features/plugin.html`
book/compiler-plugins.html:39: broken link - `unstable-book/language-features/plugin.html`
checked links in: 5.1s
number of HTML files scanned: 34812
number of HTML redirects found: 10550
number of links checked: 2992556
number of links ignored due to external: 59231
number of links ignored due to exceptions: 19
number of intra doc links ignored: 7
errors found: 2
found some broken links

[carols10cents]

Sounds good, thank you so much!