rustdoc: remove unportable markdown lint and old parser

Follow up https://github.com/rust-lang/rust/pull/127127

tests/rustdoc-ui/unportable-markdown.rs

@@ -1,62 +0,0 @@
-// https://internals.rust-lang.org/t/proposal-migrate-the-syntax-of-rustdoc-markdown-footnotes-to-be-compatible-with-the-syntax-used-in-github/18929
-//
-// A series of test cases for CommonMark corner cases that pulldown-cmark 0.11 fixes.
-//
-// This version of the lint is targeted at two especially-common cases where docs got broken.
-// Other differences in parsing should not warn.
-#![allow(rustdoc::broken_intra_doc_links)]
-#![deny(rustdoc::unportable_markdown)]
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/654>
-///
-/// Test footnote [^foot].
-///
-/// [^foot]: This is nested within the footnote now, but didn't used to be.
-///
-///     This is a multi-paragraph footnote.
-pub struct GfmFootnotes;
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/773>
-///
-/// test [^foo][^bar]
-///
-/// [^foo]: test
-/// [^bar]: test2
-pub struct FootnoteSmashedName;
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/829>
-///
-/// - _t
-///   # test
-///   t_
-pub struct NestingCornerCase;
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/650>
-///
-/// *~~__emphasis strike strong__~~* ~~*__strike emphasis strong__*~~
-pub struct Emphasis1;
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/732>
-///
-/// |
-/// |
-pub struct NotEnoughTable;
-
-/// <https://github.com/pulldown-cmark/pulldown-cmark/pull/675>
-///
-/// foo
-/// >bar
-//~^ ERROR unportable markdown
-pub struct BlockQuoteNoSpace;
-
-/// Negative test.
-///
-/// foo
-/// > bar
-pub struct BlockQuoteSpace;
-
-/// Negative test.
-///
-/// >bar
-/// baz
-pub struct BlockQuoteNoSpaceStart;

tests/rustdoc-ui/unportable-markdown.stderr

@@ -1,23 +0,0 @@
-error: unportable markdown
-  --> $DIR/unportable-markdown.rs:48:5
-   |
-LL | /// >bar
-   |     ^
-   |
-   = help: confusing block quote with no space after the `>` marker
-note: the lint level is defined here
-  --> $DIR/unportable-markdown.rs:8:9
-   |
-LL | #![deny(rustdoc::unportable_markdown)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-help: if the quote is intended, add a space
-   |
-LL | /// > bar
-   |      +
-help: if it should not be a quote, escape it
-   |
-LL | /// \>bar
-   |     +
-
-error: aborting due to 1 previous error
-