Remove piece of incomplete documentation

[andreabedini]

Virtual modules are discussed right below in the library section.

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[andreabedini]

I need to check if this is library pkg field or not.

[BinderDavid]

Is anything preventing this from being merged? This TODO still exists in the docs, and the field is already documented in the package description section. I don't want to open another PR if this one is already open.

[ulysses4ever]

@andreabedini do you want to connect this to the Ready state (it's currently Draft)? Also, there's a docs build failure?

[ulysses4ever]

@mergify rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[ulysses4ever]

Looks like this part of the output is related:

/home/runner/work/cabal/cabal/doc/buildinfo-fields-reference.rst:507: WARNING: cabal:pkg-field reference target not found: virtual-modules

[BinderDavid]

The section was removed in #9400