mk: Stop generating docs for deprecated crates

These crates are all deprecated for their rust-lang/$crate equivalents and by
generating docs we're generating broken links. The documentation for these
crates are generated out-of-tree and are managed separately, so we're not losing
the documentation altogether, just the links from the main distribution's docs.

Closes #20096
auto merge of #20101 : alexcrichton/rust/issue-20096, r=aturon

These crates are all deprecated for their rust-lang/$crate equivalents and by
generating docs we're generating broken links. The documentation for these
crates are generated out-of-tree and are managed separately, so we're not losing
the documentation altogether, just the links from the main distribution's docs.

Closes #20096